fix(events): Handle nullable label and milestone in resource events

GitLab returns null for the label/milestone fields on resource_label_events
and resource_milestone_events when the referenced label or milestone has
been deleted. This caused deserialization failures during sync.

- Add migration 012 to recreate both event tables with nullable
  label_name, milestone_title, and milestone_id columns (SQLite
  requires table recreation to alter NOT NULL constraints)
- Change GitLabLabelEvent.label and GitLabMilestoneEvent.milestone
  to Option<> in the Rust types
- Update upsert functions to pass through None values correctly
- Add tests for null label and null milestone deserialization

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

src/core/db.rs

@@ -39,6 +39,10 @@ const MIGRATIONS: &[(&str, &str)] = &[
         "011",
         include_str!("../../migrations/011_resource_events.sql"),
     ),
+    (
+        "012",
+        include_str!("../../migrations/012_nullable_label_milestone.sql"),
+    ),
 ];
 
 /// Create a database connection with production-grade pragmas.