perf(ingestion): replace per-row INSERT loops with chunked batch INSERTs

The issue and MR ingestion paths previously inserted labels, assignees,
and reviewers one row at a time inside a transaction. For entities with
many labels or assignees, this issued N separate SQLite statements where
a single multi-row INSERT suffices.

Replace the per-row loops with batch INSERT functions that build a
single `INSERT OR IGNORE ... VALUES (?1,?2),(?1,?3),...` statement per
chunk. Chunks are capped at 400 rows (BATCH_LINK_ROWS_MAX) to stay
comfortably below SQLite's default 999 bind-parameter limit.

Affected paths:
- issues.rs: link_issue_labels_batch_tx, insert_issue_assignees_batch_tx
- merge_requests.rs: insert_mr_labels_batch_tx,
  insert_mr_assignees_batch_tx, insert_mr_reviewers_batch_tx

New tests verify deduplication (OR IGNORE), multi-chunk correctness,
and equivalence with the old per-row approach. A perf benchmark
(bench_issue_assignee_insert_individual_vs_batch) demonstrates the
speedup across representative assignee set sizes.

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

src/ingestion/issues.rs

@@ -36,6 +36,9 @@ struct SyncCursor {
     tie_breaker_id: Option<i64>,
 }
 
+// Keep comfortably below SQLite's default 999 bind-parameter limit.
+const BATCH_LINK_ROWS_MAX: usize = 400;
+
 pub async fn ingest_issues(
     conn: &Connection,
     client: &GitLabClient,
@@ -252,22 +255,19 @@ fn process_issue_in_transaction(
         [local_issue_id],
     )?;
 
+    let mut label_ids = Vec::with_capacity(label_names.len());
     for label_name in label_names {
         let label_id = upsert_label_tx(tx, project_id, label_name, &mut labels_created)?;
-        link_issue_label_tx(tx, local_issue_id, label_id)?;
+        label_ids.push(label_id);
     }
+    link_issue_labels_batch_tx(tx, local_issue_id, &label_ids)?;
 
     tx.execute(
         "DELETE FROM issue_assignees WHERE issue_id = ?",
         [local_issue_id],
     )?;
 
-    for username in assignee_usernames {
-        tx.execute(
-            "INSERT OR IGNORE INTO issue_assignees (issue_id, username) VALUES (?, ?)",
-            (local_issue_id, username),
-        )?;
-    }
+    insert_issue_assignees_batch_tx(tx, local_issue_id, assignee_usernames)?;
 
     Ok(labels_created)
 }
@@ -296,11 +296,65 @@ fn upsert_label_tx(
     Ok(id)
 }
 
-fn link_issue_label_tx(tx: &Transaction<'_>, issue_id: i64, label_id: i64) -> Result<()> {
-    tx.execute(
-        "INSERT OR IGNORE INTO issue_labels (issue_id, label_id) VALUES (?, ?)",
-        (issue_id, label_id),
-    )?;
+fn link_issue_labels_batch_tx(
+    tx: &Transaction<'_>,
+    issue_id: i64,
+    label_ids: &[i64],
+) -> Result<()> {
+    if label_ids.is_empty() {
+        return Ok(());
+    }
+
+    for chunk in label_ids.chunks(BATCH_LINK_ROWS_MAX) {
+        let placeholders = (0..chunk.len())
+            .map(|idx| format!("(?1, ?{})", idx + 2))
+            .collect::<Vec<_>>()
+            .join(", ");
+        let sql = format!(
+            "INSERT OR IGNORE INTO issue_labels (issue_id, label_id) VALUES {}",
+            placeholders
+        );
+
+        let mut params: Vec<&dyn rusqlite::types::ToSql> = Vec::with_capacity(chunk.len() + 1);
+        params.push(&issue_id);
+        for label_id in chunk {
+            params.push(label_id);
+        }
+
+        tx.execute(&sql, params.as_slice())?;
+    }
+
+    Ok(())
+}
+
+fn insert_issue_assignees_batch_tx(
+    tx: &Transaction<'_>,
+    issue_id: i64,
+    usernames: &[String],
+) -> Result<()> {
+    if usernames.is_empty() {
+        return Ok(());
+    }
+
+    for chunk in usernames.chunks(BATCH_LINK_ROWS_MAX) {
+        let placeholders = (0..chunk.len())
+            .map(|idx| format!("(?1, ?{})", idx + 2))
+            .collect::<Vec<_>>()
+            .join(", ");
+        let sql = format!(
+            "INSERT OR IGNORE INTO issue_assignees (issue_id, username) VALUES {}",
+            placeholders
+        );
+
+        let mut params: Vec<&dyn rusqlite::types::ToSql> = Vec::with_capacity(chunk.len() + 1);
+        params.push(&issue_id);
+        for username in chunk {
+            params.push(username);
+        }
+
+        tx.execute(&sql, params.as_slice())?;
+    }
+
     Ok(())
 }