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/lms/README.md

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+# LMS (Lightweight Music Server)
+
+## Overview
+
+Self-hosted music streaming software with comprehensive metadata support. Access your music collection from anywhere using a web interface.
+
+## Key Features
+
+- **Stars**: 1,569
+- **API**: Subsonic/OpenSubsonic
+- **Language**: C++
+- **Metadata**: MusicBrainz identifiers, artist relationships, release types
+- **License**: GPL-3.0
+
+## Source
+
+| Resource | URL |
+|----------|-----|
+| **Repository** | https://github.com/epoupon/lms |
+| **AUR Package** | https://aur.archlinux.org/packages/lms |
+
+## Metadata Features
+
+- Multi-valued tags: genre, mood, artists
+- Artist relationships: composer, conductor, lyricist, mixer, performer, producer, remixer
+- Release types: album, single, EP, compilation, live
+- Release groups (different versions: remasters, reissues)
+- MusicBrainz identifier support
+- ListenBrainz integration
+
+## Supported Tags
+
+```
+# MusicBrainz IDs
+musicbrainz_composerid, musicbrainz_conductorid
+musicbrainz_lyricistid, musicbrainz_mixerid
+musicbrainz_producerid, musicbrainz_remixerid
+
+# Sort order
+albumartistssort, composerssort, conductorssort
+lyricistssort, mixerssort, producerssort, remixerssort
+```
+
+## Artist Info Folder
+
+Supports Kodi-style artist information folders:
+- `artist.nfo` files for biography, sort name, MBID
+- Custom artist images
+
+## Self-Hosting
+
+```bash
+# Build from source (requires C++ compiler)
+git clone https://github.com/epoupon/lms.git
+cd lms
+mkdir build && cd build
+cmake ..
+make -j$(nproc)
+
+# Or use Docker
+docker pull epoupon/lms
+```
+
+## Notes
+
+- Very complete metadata support
+- Handles duplicate artist/release names via MBIDs
+- Lightweight C++ implementation
+- Active development (3 open issues)