feat(who): expand expert + overlap queries with mr_file_changes and mr_reviewers

Chain: bd-jec (config flag) -> bd-2yo (fetch MR diffs) -> bd-3qn6 (rewrite who queries)

- Add fetch_mr_file_changes config option and --no-file-changes CLI flag
- Add GitLab MR diffs API fetch pipeline with watermark-based sync
- Create migration 020 for diffs_synced_for_updated_at watermark column
- Rewrite query_expert() and query_overlap() to use 4-signal UNION ALL:
  DiffNote reviewers, DiffNote MR authors, file-change authors, file-change reviewers
- Deduplicate across signal types via COUNT(DISTINCT CASE WHEN ... THEN mr_id END)
- Add insert_file_change test helper, 8 new who tests, all 397 tests pass
- Also includes: list performance migration 019, autocorrect module, README updates

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ingestion/mr_diffs.rs

@@ -0,0 +1,268 @@
+use rusqlite::Connection;
+use tracing::debug;
+
+use crate::core::error::Result;
+use crate::gitlab::types::GitLabMrDiff;
+
+/// Derive the change type from GitLab's boolean flags.
+fn derive_change_type(diff: &GitLabMrDiff) -> &'static str {
+    if diff.new_file {
+        "added"
+    } else if diff.renamed_file {
+        "renamed"
+    } else if diff.deleted_file {
+        "deleted"
+    } else {
+        "modified"
+    }
+}
+
+/// Replace all file change records for a given MR with the provided diffs.
+/// Uses DELETE+INSERT (simpler than UPSERT for array replacement).
+pub fn upsert_mr_file_changes(
+    conn: &Connection,
+    mr_local_id: i64,
+    project_id: i64,
+    diffs: &[GitLabMrDiff],
+) -> Result<usize> {
+    conn.execute(
+        "DELETE FROM mr_file_changes WHERE merge_request_id = ?1",
+        [mr_local_id],
+    )?;
+
+    let mut stmt = conn.prepare_cached(
+        "INSERT INTO mr_file_changes (merge_request_id, project_id, old_path, new_path, change_type) \
+         VALUES (?1, ?2, ?3, ?4, ?5)",
+    )?;
+
+    let mut inserted = 0;
+    for diff in diffs {
+        let old_path = if diff.renamed_file {
+            Some(diff.old_path.as_str())
+        } else {
+            None
+        };
+        let change_type = derive_change_type(diff);
+
+        stmt.execute(rusqlite::params![
+            mr_local_id,
+            project_id,
+            old_path,
+            diff.new_path,
+            change_type,
+        ])?;
+        inserted += 1;
+    }
+
+    if inserted > 0 {
+        debug!(inserted, mr_local_id, "Stored MR file changes");
+    }
+
+    Ok(inserted)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::core::db::{create_connection, run_migrations};
+    use std::path::Path;
+
+    fn setup() -> Connection {
+        let conn = create_connection(Path::new(":memory:")).unwrap();
+        run_migrations(&conn).unwrap();
+
+        // Insert a test project
+        conn.execute(
+            "INSERT INTO projects (gitlab_project_id, path_with_namespace, web_url) VALUES (1, 'group/repo', 'https://gitlab.com/group/repo')",
+            [],
+        ).unwrap();
+
+        // Insert a test MR
+        conn.execute(
+            "INSERT INTO merge_requests (gitlab_id, iid, project_id, title, state, draft, source_branch, target_branch, author_username, created_at, updated_at, last_seen_at) \
+             VALUES (100, 1, 1, 'Test MR', 'merged', 0, 'feature', 'main', 'testuser', 1000, 2000, 3000)",
+            [],
+        ).unwrap();
+
+        conn
+    }
+
+    #[test]
+    fn test_derive_change_type_added() {
+        let diff = GitLabMrDiff {
+            old_path: String::new(),
+            new_path: "src/new.rs".to_string(),
+            new_file: true,
+            renamed_file: false,
+            deleted_file: false,
+        };
+        assert_eq!(derive_change_type(&diff), "added");
+    }
+
+    #[test]
+    fn test_derive_change_type_renamed() {
+        let diff = GitLabMrDiff {
+            old_path: "src/old.rs".to_string(),
+            new_path: "src/new.rs".to_string(),
+            new_file: false,
+            renamed_file: true,
+            deleted_file: false,
+        };
+        assert_eq!(derive_change_type(&diff), "renamed");
+    }
+
+    #[test]
+    fn test_derive_change_type_deleted() {
+        let diff = GitLabMrDiff {
+            old_path: "src/gone.rs".to_string(),
+            new_path: "src/gone.rs".to_string(),
+            new_file: false,
+            renamed_file: false,
+            deleted_file: true,
+        };
+        assert_eq!(derive_change_type(&diff), "deleted");
+    }
+
+    #[test]
+    fn test_derive_change_type_modified() {
+        let diff = GitLabMrDiff {
+            old_path: "src/lib.rs".to_string(),
+            new_path: "src/lib.rs".to_string(),
+            new_file: false,
+            renamed_file: false,
+            deleted_file: false,
+        };
+        assert_eq!(derive_change_type(&diff), "modified");
+    }
+
+    #[test]
+    fn test_upsert_inserts_file_changes() {
+        let conn = setup();
+        let diffs = [
+            GitLabMrDiff {
+                old_path: String::new(),
+                new_path: "src/new.rs".to_string(),
+                new_file: true,
+                renamed_file: false,
+                deleted_file: false,
+            },
+            GitLabMrDiff {
+                old_path: "src/lib.rs".to_string(),
+                new_path: "src/lib.rs".to_string(),
+                new_file: false,
+                renamed_file: false,
+                deleted_file: false,
+            },
+        ];
+
+        let inserted = upsert_mr_file_changes(&conn, 1, 1, &diffs).unwrap();
+        assert_eq!(inserted, 2);
+
+        let count: i64 = conn
+            .query_row(
+                "SELECT COUNT(*) FROM mr_file_changes WHERE merge_request_id = 1",
+                [],
+                |r| r.get(0),
+            )
+            .unwrap();
+        assert_eq!(count, 2);
+    }
+
+    #[test]
+    fn test_upsert_replaces_existing() {
+        let conn = setup();
+        let diffs_v1 = [GitLabMrDiff {
+            old_path: String::new(),
+            new_path: "src/old.rs".to_string(),
+            new_file: true,
+            renamed_file: false,
+            deleted_file: false,
+        }];
+        upsert_mr_file_changes(&conn, 1, 1, &diffs_v1).unwrap();
+
+        let diffs_v2 = [
+            GitLabMrDiff {
+                old_path: "src/a.rs".to_string(),
+                new_path: "src/a.rs".to_string(),
+                new_file: false,
+                renamed_file: false,
+                deleted_file: false,
+            },
+            GitLabMrDiff {
+                old_path: "src/b.rs".to_string(),
+                new_path: "src/b.rs".to_string(),
+                new_file: false,
+                renamed_file: false,
+                deleted_file: false,
+            },
+        ];
+        let inserted = upsert_mr_file_changes(&conn, 1, 1, &diffs_v2).unwrap();
+        assert_eq!(inserted, 2);
+
+        let count: i64 = conn
+            .query_row(
+                "SELECT COUNT(*) FROM mr_file_changes WHERE merge_request_id = 1",
+                [],
+                |r| r.get(0),
+            )
+            .unwrap();
+        assert_eq!(count, 2);
+
+        // The old "src/old.rs" should be gone
+        let old_count: i64 = conn
+            .query_row(
+                "SELECT COUNT(*) FROM mr_file_changes WHERE new_path = 'src/old.rs'",
+                [],
+                |r| r.get(0),
+            )
+            .unwrap();
+        assert_eq!(old_count, 0);
+    }
+
+    #[test]
+    fn test_renamed_stores_old_path() {
+        let conn = setup();
+        let diffs = [GitLabMrDiff {
+            old_path: "src/old_name.rs".to_string(),
+            new_path: "src/new_name.rs".to_string(),
+            new_file: false,
+            renamed_file: true,
+            deleted_file: false,
+        }];
+
+        upsert_mr_file_changes(&conn, 1, 1, &diffs).unwrap();
+
+        let (old_path, change_type): (Option<String>, String) = conn
+            .query_row(
+                "SELECT old_path, change_type FROM mr_file_changes WHERE new_path = 'src/new_name.rs'",
+                [],
+                |r| Ok((r.get(0)?, r.get(1)?)),
+            )
+            .unwrap();
+        assert_eq!(old_path.as_deref(), Some("src/old_name.rs"));
+        assert_eq!(change_type, "renamed");
+    }
+
+    #[test]
+    fn test_non_renamed_has_null_old_path() {
+        let conn = setup();
+        let diffs = [GitLabMrDiff {
+            old_path: "src/lib.rs".to_string(),
+            new_path: "src/lib.rs".to_string(),
+            new_file: false,
+            renamed_file: false,
+            deleted_file: false,
+        }];
+
+        upsert_mr_file_changes(&conn, 1, 1, &diffs).unwrap();
+
+        let old_path: Option<String> = conn
+            .query_row(
+                "SELECT old_path FROM mr_file_changes WHERE new_path = 'src/lib.rs'",
+                [],
+                |r| r.get(0),
+            )
+            .unwrap();
+        assert!(old_path.is_none());
+    }
+}