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

buf.gen.yaml

@@ -0,0 +1,13 @@
+version: v2
+managed:
+  enabled: true
+  override:
+    - file_option: go_package_prefix
+      value: github.com/metadata-agregator/pkg/gen
+plugins:
+  - remote: buf.build/protocolbuffers/go
+    out: pkg/gen
+    opt: paths=source_relative
+  - remote: buf.build/grpc/go
+    out: pkg/gen
+    opt: paths=source_relative