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/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};