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>
