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/lidarr-metadata-api/README.md

@@ -0,0 +1,54 @@
+# Lidarr Metadata API
+
+## Overview
+
+Custom metadata API that powers Lidarr (music collection manager). Built on top of MusicBrainz with enhanced artist/album data.
+
+## Key Features
+
+- **Purpose**: Metadata backend for Lidarr
+- **Data Source**: MusicBrainz PostgreSQL + Solr
+- **API**: REST
+- **License**: GPL-3.0
+
+## Source
+
+| Resource | URL |
+|----------|-----|
+| **Repository** | https://github.com/Lidarr/LidarrAPI.Metadata |
+| **Lidarr Main** | https://github.com/Lidarr/Lidarr |
+| **Documentation** | https://wiki.servarr.com/lidarr |
+
+## Architecture
+
+Requires:
+- MusicBrainz PostgreSQL database
+- Solr search server
+
+```
+docker-compose.yml         # Base services (MusicBrainz DB, Solr)
+docker-compose.dev.yml     # Dev mode (exposed ports)
+docker-compose.prod.yml    # Production (metadata service in Docker)
+```
+
+## Self-Hosting
+
+```bash
+git clone https://github.com/Lidarr/LidarrAPI.Metadata.git
+cd LidarrAPI.Metadata
+
+# Start with Docker Compose
+docker-compose -f docker-compose.yml -f docker-compose.prod.yml up
+
+# Or run directly
+python server.py
+# or
+lidarr-metadata-server
+```
+
+## Notes
+
+- Powers the Lidarr ecosystem (music management for *arr stack)
+- Enhanced MusicBrainz data with better album matching
+- Community-hosted instance at `api.musicinfo.pro`
+- Requires significant resources (~350GB for full MusicBrainz mirror)