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/issue.rs

@@ -24,8 +24,8 @@ pub struct IssueRow {
     pub created_at: i64, // ms epoch UTC
     pub updated_at: i64, // ms epoch UTC
     pub web_url: String,
-    pub due_date: Option<String>,           // YYYY-MM-DD
-    pub milestone_title: Option<String>,    // Denormalized for quick display
+    pub due_date: Option<String>,        // YYYY-MM-DD
+    pub milestone_title: Option<String>, // Denormalized for quick display
 }
 
 /// Local schema representation of a milestone row.
@@ -62,11 +62,8 @@ pub fn transform_issue(issue: GitLabIssue) -> Result<IssueWithMetadata, Transfor
     let created_at = parse_timestamp(&issue.created_at)?;
     let updated_at = parse_timestamp(&issue.updated_at)?;
 
-    let assignee_usernames: Vec<String> = issue
-        .assignees
-        .iter()
-        .map(|a| a.username.clone())
-        .collect();
+    let assignee_usernames: Vec<String> =
+        issue.assignees.iter().map(|a| a.username.clone()).collect();
 
     let milestone_title = issue.milestone.as_ref().map(|m| m.title.clone());
 
@@ -252,7 +249,10 @@ mod tests {
         assert_eq!(milestone.description, Some("First release".to_string()));
         assert_eq!(milestone.state, Some("active".to_string()));
         assert_eq!(milestone.due_date, Some("2024-02-01".to_string()));
-        assert_eq!(milestone.web_url, Some("https://gitlab.example.com/-/milestones/5".to_string()));
+        assert_eq!(
+            milestone.web_url,
+            Some("https://gitlab.example.com/-/milestones/5".to_string())
+        );
     }
 
     #[test]