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/gonic/README.md

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+# gonic
+
+## Overview
+
+Free-software Subsonic server API implementation. Music streaming server written in Go, lightweight and suitable for Raspberry Pi.
+
+## Key Features
+
+- **API**: Subsonic/OpenSubsonic
+- **Language**: Go
+- **Metadata**: Embedded tags, Last.fm, ListenBrainz
+- **Transcoding**: On-the-fly with ffmpeg
+- **License**: GPL-3.0
+
+## Source
+
+| Resource | URL |
+|----------|-----|
+| **Repository** | https://github.com/sentriz/gonic |
+| **Docker Hub** | https://hub.docker.com/r/sentriz/gonic |
+
+## Key Features
+
+- Browsing by folder (keeps tree intact) or by tags
+- Multi-valued tags support (genres, album artists)
+- On-the-fly transcoding and caching (requires ffmpeg)
+- Jukebox mode (server-side playback)
+- Podcast support
+- Last.fm and ListenBrainz scrobbling
+- Artist similarities and biographies from Last.fm
+- Web interface for configuration
+
+## Tag Support
+
+```
+# Multi-value tag modes
+GONIC_MULTI_VALUE_MODE=multi   # Explicit multi-value fields (genres, album_artists)
+GONIC_MULTI_VALUE_MODE=delim   # Delimiter-separated values
+```
+
+## Self-Hosting
+
+```bash
+docker run -d \
+  -p 4747:80 \
+  -v /path/to/music:/music:ro \
+  -v /path/to/data:/data \
+  -v /path/to/podcasts:/podcasts \
+  -v /path/to/cache:/cache \
+  sentriz/gonic
+```
+
+## Tested Clients
+
+- airsonic-refix, amperfy, symfonium, dsub
+- jamstash, music-assistant, subsonic.el
+- sublime music, soundwaves, stmp, termsonic
+- tempus, strawberry, ultrasonic
+
+## Notes
+
+- Lightweight Go implementation
+- MusicBrainz Picard / Beets / wrtag compatible tags
+- ARM images available for Raspberry Pi
+- Active development