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/navidrome/README.md

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+# Navidrome
+
+## Overview
+
+Modern music server and streamer compatible with Subsonic/Airsonic clients. Lightweight, fast, and self-contained with web-based UI.
+
+## Key Features
+
+- **Popularity**: Very high (one of the most popular self-hosted music servers)
+- **API**: OpenSubsonic v1.16.1
+- **Metadata**: Library scans + Last.fm integration
+- **Language**: Go
+- **License**: GPL-3.0
+
+## Source
+
+| Resource | URL |
+|----------|-----|
+| **Repository** | https://github.com/navidrome/navidrome |
+| **Website** | https://navidrome.org |
+| **Documentation** | https://www.navidrome.org/docs |
+
+## Key Features
+
+- Very low resource usage (Go binary)
+- Handles large libraries (tested with 900K+ songs)
+- Multi-user support with individual settings
+- Transcoding on-the-fly
+- Last.fm scrobbling
+- Lyrics support (embedded and `.lrc` files)
+- Multiple themes
+
+## API Endpoints
+
+```bash
+# Subsonic API
+GET /rest/getArtists
+GET /rest/getArtist?id={artistId}
+GET /rest/getAlbum?id={albumId}
+GET /rest/getSong?id={songId}
+GET /rest/search3?query={query}
+```
+
+## Self-Hosting
+
+```bash
+# Docker
+docker run -d \
+  --name navidrome \
+  -p 4533:4533 \
+  -v /path/to/music:/music:ro \
+  -v /path/to/data:/data \
+  deluan/navidrome
+
+# Or download binary
+# https://github.com/navidrome/navidrome/releases
+```
+
+## Notes
+
+- Compatible with many Subsonic clients (DSub, Symfonium, Ultrasonic, etc.)
+- Very lightweight (suitable for Raspberry Pi)
+- Web UI included
+- Primary focus is streaming, but provides comprehensive metadata API