Add async worker and feature flag for stale todo resolution

Goal

Add async processing capability and feature flag infrastructure. Still no user-facing changes.

Related to #20637 (closed)

Depends on: Phase 1a (link after creation)

Implementation Details

1. Sidekiq Worker

Location: app/workers/merge_requests/resolve_stale_todos_worker.rb

• Idempotent (safe to retry)
• Low urgency (doesn't block user actions)
• Graceful error handling for missing records

2. Feature Flag

Location: config/feature_flags/development/auto_resolve_stale_mr_todos.yml

• Type: development (will change to ops during rollout)
• Scope: Project-level (allows per-project enablement)
• Default: false (disabled by default)

Test Coverage Required

Location: spec/workers/merge_requests/resolve_stale_todos_worker_spec.rb

• Successfully calls TodoService method
• Passes correct parameters to service
• Handles missing merge request gracefully (no error)
• Handles missing user gracefully (no error)
• Is idempotent (can run multiple times safely)
• Has correct worker attributes (urgency, category, etc.)

Files to Create

• app/workers/merge_requests/resolve_stale_todos_worker.rb
• spec/workers/merge_requests/resolve_stale_todos_worker_spec.rb
• config/feature_flags/development/auto_resolve_stale_mr_todos.yml

Acceptance Criteria

• Worker successfully calls TodoService method from Phase 1a
• Worker handles missing records gracefully
• Worker is idempotent
• Feature flag is registered correctly
• 100% test coverage for worker
• All CI jobs pass

Estimated Effort: 2 days