feat: initial implementation of metadata aggregator

- gRPC service with MusicBrainz provider
- PostgreSQL schema with migrations
- Service layer with database-first caching
- Repository pattern for data access
- YAML configuration support
- Research documentation for 17 music metadata projects

database/compose.yaml

@@ -0,0 +1,23 @@
+services:
+  postgres:
+    image: postgres:16-alpine
+    container_name: metadata-postgres
+    environment:
+      POSTGRES_USER: metadata
+      POSTGRES_PASSWORD: metadata
+      POSTGRES_DB: metadata
+    ports:
+      - "5432:5432"
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
+      - ./migrations:/docker-entrypoint-initdb.d:ro
+      - ./postgresql.conf:/etc/postgresql/postgresql.conf:ro
+    command: postgres -c config_file=/etc/postgresql/postgresql.conf
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U metadata -d metadata"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+
+volumes:
+  postgres_data: