[Feature]: Estate: Remote Entity Metadata table and repository

Parent PRD

#143

What to build

Create the shared remote_entity_metadata DB table and repository that tracks which local entities are replicas of remote ones. This is the core data structure enabling remote awareness across the entire app.

• New DB model: RemoteEntityMetadataDb with columns: entity_type, entity_id, source_instance_id, source_entity_id, source_instance_name, source_instance_url, last_synced_at, is_stale, stale_since
• Repository with CRUD, query by source instance, query by entity type, bulk stale-marking
• Composite unique constraint on (entity_type, entity_id)

See "Remote Entity Tracking" section of the parent PRD.

Acceptance criteria

• RemoteEntityMetadataDb model created with all specified columns
• Repository supports: create, update, get by entity, query by source instance, query by entity type, mark stale, bulk mark stale
• Composite unique constraint on (entity_type, entity_id)
• Tests: test_remote_entity_metadata.py covering CRUD, stale transitions, queries

Blocked by

None - can start immediately

User stories addressed

• User story 17

Start with the “Remote Entity Tracking” section of parent PRD #143, then locate the database model and repository entry points for the new remote_entity_metadata table. Implement the listed CRUD, query, stale-marking, and uniqueness behavior, and use test_remote_entity_metadata.py to cover CRUD, stale transitions, and queries.