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

docs/research/music-metadata-api/README.md

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+# Music Metadata API
+
+## Overview
+
+A high-performance metadata enrichment API for music servers, backed by SQLite databases containing 256 million tracks. Written in Go with OpenAPI 3.1 spec.
+
+## Key Features
+
+- **Coverage**: 256M tracks in SQLite databases
+- **Performance**: 100 req/s with burst capacity of 200
+- **Batch API**: Up to 400 entities per request
+- **Data**: Images, genres, labels, copyright, release dates, ISRC
+- **License**: MIT
+
+## Source
+
+| Resource | URL |
+|----------|-----|
+| **Repository** | https://github.com/Aunali321/music-metadata-api |
+| **API Docs** | Swagger UI at `/docs` endpoint |
+
+## API Endpoints
+
+| Endpoint | Description |
+|----------|-------------|
+| `POST /batch/lookup` | Batch lookup (up to 400 entities) |
+| `GET /lookup/isrc/{isrc}` | Lookup tracks by ISRC |
+| `GET /lookup/track/{id}` | Lookup track by ID |
+| `GET /lookup/artist/{id}` | Lookup artist by ID |
+| `GET /lookup/album/{id}` | Lookup album by ID |
+| `GET /lookup/album/{id}/tracks` | Get all tracks in album |
+| `GET /search/track?q=&limit=` | Search tracks by name |
+| `GET /search/artist?q=&limit=` | Search artists by name |
+
+## Self-Hosting
+
+```bash
+# Docker
+docker run -p 8080:8080 \
+  -v /path/to/databases:/data:ro \
+  ghcr.io/aunali321/music-metadata-api:latest
+
+# Or build locally (requires Go 1.24+)
+./metadata-api -db /path/to/main_database.sqlite3
+```
+
+## Notes
+
+- **Requires separate SQLite databases** (not included in repo)
+- Independent open-source project (not affiliated with Spotify)
+- Best for high-volume bulk lookups
+- OpenAPI 3.1 spec available at `/openapi.yaml`