feat(communities): Add PostgreSQL repository with atomic operations master
Database schema: - communities table with pg_trgm indexes for fuzzy search - community_subscriptions with composite index for lookups - community_memberships with reputation tracking - community_moderation (V2 prepared) Repository features: - Atomic SubscribeWithCount/UnsubscribeWithCount transactions - Idempotent operations for Jetstream replay safety - Full-text search with relevance filtering (>0.2 threshold) - Pagination and filtering support - Proper error handling with domain error mapping Performance optimizations: - Composite index on (user_did, community_did) for subscription lookups - GIN indexes for trigram similarity search - GREATEST(0, count - 1) prevents negative counts