feat(transformers): Add MR transformer and polymorphic discussion support

Introduces NormalizedMergeRequest transformer and updates discussion
normalization to handle both issue and MR discussions polymorphically.

New transformers:
- NormalizedMergeRequest: Transforms API MergeRequest to database row,
  extracting labels/assignees/reviewers into separate collections for
  junction table insertion. Handles draft detection, detailed_merge_status
  preference over deprecated merge_status, and merge_user over merged_by.

Discussion transformer updates:
- NormalizedDiscussion now takes noteable_type ("Issue" | "MergeRequest")
  and noteable_id for polymorphic FK binding
- normalize_discussions_for_issue(): Convenience wrapper for issues
- normalize_discussions_for_mr(): Convenience wrapper for MRs
- DiffNote position fields (type, line_range, SHA triplet) now extracted
  from API position object for code review context

Design decisions:
- Transformer returns (normalized_item, labels, assignees, reviewers)
  tuple for efficient batch insertion without re-querying
- Timestamps converted to ms epoch for SQLite storage consistency
- Optional fields use map() chains for clean null handling

The polymorphic discussion approach allows reusing the same discussions
and notes tables for both issues and MRs, with noteable_type + FK
determining the parent relationship.

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

src/gitlab/transformers/mod.rs

@@ -2,6 +2,13 @@
 
 pub mod discussion;
 pub mod issue;
+pub mod merge_request;
 
-pub use discussion::{NormalizedDiscussion, NormalizedNote, NoteableRef, transform_discussion, transform_notes};
+pub use discussion::{
+    NormalizedDiscussion, NormalizedNote, NoteableRef, transform_discussion,
+    transform_mr_discussion, transform_notes, transform_notes_with_diff_position,
+};
 pub use issue::{IssueRow, IssueWithMetadata, MilestoneRow, transform_issue};
+pub use merge_request::{
+    MergeRequestWithMetadata, NormalizedMergeRequest, transform_merge_request,
+};