fix(discussion): Make NormalizedDiscussion polymorphic for MR support

This is a P0 fix from the CP1-CP2 alignment audit. The original
NormalizedDiscussion struct had issue_id as a non-optional i64 and
hardcoded noteable_type to "Issue", making it incompatible with merge
request discussions even though the database schema already supports
both via nullable columns and a CHECK constraint.

Changes:
- Add NoteableRef enum with Issue(i64) and MergeRequest(i64) variants
  to provide compile-time safety against mixing up issue vs MR IDs
- Change NormalizedDiscussion.issue_id from i64 to Option<i64>
- Add NormalizedDiscussion.merge_request_id: Option<i64>
- Update transform_discussion() signature to take NoteableRef instead
  of local_issue_id, deriving issue_id/merge_request_id/noteable_type
  from the enum variant
- Update upsert_discussion() SQL to include merge_request_id column
  (now 12 parameters instead of 11)
- Export NoteableRef from transformers module
- Add test for MergeRequest discussion transformation
- Update all existing tests to use NoteableRef::Issue(id)

The database schema (migration 002) was forward-thinking and already
supports both issue_id and merge_request_id as nullable columns with
a CHECK constraint. This change prepares the application layer for
CP2 merge request support without requiring any migrations.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/gitlab/transformers/discussion.rs

@@ -5,13 +5,22 @@ use chrono::DateTime;
 use crate::core::time::now_ms;
 use crate::gitlab::types::{GitLabDiscussion, GitLabNote};
 
+/// Reference to the parent noteable (Issue or MergeRequest).
+/// Uses an enum to prevent accidentally mixing up issue vs MR IDs at compile time.
+#[derive(Debug, Clone, Copy)]
+pub enum NoteableRef {
+    Issue(i64),
+    MergeRequest(i64),
+}
+
 /// Normalized discussion for local storage.
 #[derive(Debug, Clone)]
 pub struct NormalizedDiscussion {
     pub gitlab_discussion_id: String,
     pub project_id: i64,
-    pub issue_id: i64,
-    pub noteable_type: String, // "Issue"
+    pub issue_id: Option<i64>,
+    pub merge_request_id: Option<i64>,
+    pub noteable_type: String, // "Issue" or "MergeRequest"
     pub individual_note: bool,
     pub first_note_at: Option<i64>, // min(note.created_at) in ms epoch
     pub last_note_at: Option<i64>,  // max(note.created_at) in ms epoch
@@ -55,10 +64,16 @@ fn parse_timestamp(ts: &str) -> i64 {
 pub fn transform_discussion(
     gitlab_discussion: &GitLabDiscussion,
     local_project_id: i64,
-    local_issue_id: i64,
+    noteable: NoteableRef,
 ) -> NormalizedDiscussion {
     let now = now_ms();
 
+    // Derive issue_id, merge_request_id, and noteable_type from the enum
+    let (issue_id, merge_request_id, noteable_type) = match noteable {
+        NoteableRef::Issue(id) => (Some(id), None, "Issue"),
+        NoteableRef::MergeRequest(id) => (None, Some(id), "MergeRequest"),
+    };
+
     // Compute first_note_at and last_note_at from notes
     let note_timestamps: Vec<i64> = gitlab_discussion
         .notes
@@ -86,8 +101,9 @@ pub fn transform_discussion(
     NormalizedDiscussion {
         gitlab_discussion_id: gitlab_discussion.id.clone(),
         project_id: local_project_id,
-        issue_id: local_issue_id,
-        noteable_type: "Issue".to_string(),
+        issue_id,
+        merge_request_id,
+        noteable_type: noteable_type.to_string(),
         individual_note: gitlab_discussion.individual_note,
         first_note_at,
         last_note_at,
@@ -192,18 +208,39 @@ mod tests {
             )],
         );
 
-        let result = transform_discussion(&discussion, 100, 42);
+        let result = transform_discussion(&discussion, 100, NoteableRef::Issue(42));
 
         assert_eq!(
             result.gitlab_discussion_id,
             "6a9c1750b37d513a43987b574953fceb50b03ce7"
         );
         assert_eq!(result.project_id, 100);
-        assert_eq!(result.issue_id, 42);
+        assert_eq!(result.issue_id, Some(42));
+        assert_eq!(result.merge_request_id, None);
         assert_eq!(result.noteable_type, "Issue");
         assert!(!result.individual_note);
     }
 
+    #[test]
+    fn transforms_merge_request_discussion() {
+        let discussion = make_test_discussion(
+            false,
+            vec![make_test_note(
+                1,
+                "2024-01-16T09:00:00.000Z",
+                false,
+                false,
+                false,
+            )],
+        );
+
+        let result = transform_discussion(&discussion, 100, NoteableRef::MergeRequest(99));
+
+        assert_eq!(result.issue_id, None);
+        assert_eq!(result.merge_request_id, Some(99));
+        assert_eq!(result.noteable_type, "MergeRequest");
+    }
+
     #[test]
     fn extracts_notes_array_from_discussion() {
         let discussion = make_test_discussion(
@@ -244,8 +281,8 @@ mod tests {
             )],
         );
 
-        assert!(!transform_discussion(&threaded, 100, 42).individual_note);
-        assert!(transform_discussion(&standalone, 100, 42).individual_note);
+        assert!(!transform_discussion(&threaded, 100, NoteableRef::Issue(42)).individual_note);
+        assert!(transform_discussion(&standalone, 100, NoteableRef::Issue(42)).individual_note);
     }
 
     #[test]
@@ -293,7 +330,7 @@ mod tests {
             ],
         );
 
-        let result = transform_discussion(&discussion, 100, 42);
+        let result = transform_discussion(&discussion, 100, NoteableRef::Issue(42));
 
         // first_note_at should be 09:00 (note 1)
         assert_eq!(result.first_note_at, Some(1705395600000));
@@ -314,7 +351,7 @@ mod tests {
             )],
         );
 
-        let result = transform_discussion(&discussion, 100, 42);
+        let result = transform_discussion(&discussion, 100, NoteableRef::Issue(42));
 
         assert_eq!(result.first_note_at, result.last_note_at);
         assert_eq!(result.first_note_at, Some(1705395600000));
@@ -338,8 +375,8 @@ mod tests {
             ],
         );
 
-        assert!(!transform_discussion(&not_resolvable, 100, 42).resolvable);
-        assert!(transform_discussion(&resolvable, 100, 42).resolvable);
+        assert!(!transform_discussion(&not_resolvable, 100, NoteableRef::Issue(42)).resolvable);
+        assert!(transform_discussion(&resolvable, 100, NoteableRef::Issue(42)).resolvable);
     }
 
     #[test]
@@ -374,8 +411,8 @@ mod tests {
             )],
         );
 
-        assert!(!transform_discussion(&partial, 100, 42).resolved);
-        assert!(transform_discussion(&fully_resolved, 100, 42).resolved);
-        assert!(!transform_discussion(&no_resolvable, 100, 42).resolved);
+        assert!(!transform_discussion(&partial, 100, NoteableRef::Issue(42)).resolved);
+        assert!(transform_discussion(&fully_resolved, 100, NoteableRef::Issue(42)).resolved);
+        assert!(!transform_discussion(&no_resolvable, 100, NoteableRef::Issue(42)).resolved);
     }
 }

src/gitlab/transformers/mod.rs

@@ -3,5 +3,5 @@
 pub mod discussion;
 pub mod issue;
 
-pub use discussion::{NormalizedDiscussion, NormalizedNote, transform_discussion, transform_notes};
+pub use discussion::{NormalizedDiscussion, NormalizedNote, NoteableRef, transform_discussion, transform_notes};
 pub use issue::{IssueRow, IssueWithMetadata, MilestoneRow, transform_issue};