feat(schema): Add migration 011 for resource events, entity references, and dependent fetch queue

Introduces five new tables that power temporal queries (timeline,
file-history, trace) via GitLab Resource Events APIs:

- resource_state_events: State transitions (opened/closed/reopened/merged/locked)
  with actor tracking, source commit, and source MR references
- resource_label_events: Label add/remove history per entity
- resource_milestone_events: Milestone assignment changes per entity
- entity_references: Cross-reference table (Gate 2 prep) linking
  source/target entity pairs with reference type and discovery method
- pending_dependent_fetches: Generic job queue for resource_events,
  mr_closes_issues, and mr_diffs with exponential backoff retry

All event tables enforce entity exclusivity via CHECK constraints
(exactly one of issue_id or merge_request_id must be non-NULL).
Deduplication handled via UNIQUE indexes on (gitlab_id, project_id).
FK cascades ensure cleanup when parent entities are removed.

The dependent fetch queue uses a UNIQUE constraint on
(project_id, entity_type, entity_iid, job_type) for idempotent
enqueue, with partial indexes optimizing claim and retry queries.

Registered as migration 011 in the embedded MIGRATIONS array in db.rs.

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

src/core/db.rs

@@ -47,6 +47,10 @@ const MIGRATIONS: &[(&str, &str)] = &[
         "010",
         include_str!("../../migrations/010_chunk_config.sql"),
     ),
+    (
+        "011",
+        include_str!("../../migrations/011_resource_events.sql"),
+    ),
 ];
 
 /// Create a database connection with production-grade pragmas.