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/ingestion/discussions.rs

@@ -14,7 +14,7 @@ use crate::Config;
 use crate::core::error::Result;
 use crate::core::payloads::{StorePayloadOptions, store_payload};
 use crate::gitlab::GitLabClient;
-use crate::gitlab::transformers::{transform_discussion, transform_notes};
+use crate::gitlab::transformers::{NoteableRef, transform_discussion, transform_notes};
 
 use super::issues::IssueForDiscussionSync;
 
@@ -129,8 +129,11 @@ async fn ingest_discussions_for_issue(
         )?;
 
         // Transform and store discussion
-        let normalized =
-            transform_discussion(&gitlab_discussion, local_project_id, issue.local_issue_id);
+        let normalized = transform_discussion(
+            &gitlab_discussion,
+            local_project_id,
+            NoteableRef::Issue(issue.local_issue_id),
+        );
 
         // Wrap all discussion+notes operations in a transaction for atomicity
         let tx = conn.unchecked_transaction()?;
@@ -217,10 +220,10 @@ fn upsert_discussion(
 ) -> Result<()> {
     conn.execute(
         "INSERT INTO discussions (
-            gitlab_discussion_id, project_id, issue_id, noteable_type,
+            gitlab_discussion_id, project_id, issue_id, merge_request_id, noteable_type,
             individual_note, first_note_at, last_note_at, last_seen_at,
             resolvable, resolved, raw_payload_id
-        ) VALUES (?1, ?2, ?3, ?4, ?5, ?6, ?7, ?8, ?9, ?10, ?11)
+        ) VALUES (?1, ?2, ?3, ?4, ?5, ?6, ?7, ?8, ?9, ?10, ?11, ?12)
         ON CONFLICT(project_id, gitlab_discussion_id) DO UPDATE SET
             first_note_at = excluded.first_note_at,
             last_note_at = excluded.last_note_at,
@@ -232,6 +235,7 @@ fn upsert_discussion(
             &discussion.gitlab_discussion_id,
             discussion.project_id,
             discussion.issue_id,
+            discussion.merge_request_id,
             &discussion.noteable_type,
             discussion.individual_note,
             discussion.first_note_at,