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/accentor/README.md

@@ -0,0 +1,73 @@
+# Accentor
+
+## Overview
+
+Modern self-hosted music server focusing on metadata. Provides complete control over your music with detailed metadata beyond what audio file tags support.
+
+## Key Features
+
+- **Focus**: Metadata-centric design
+- **API**: REST (Ruby on Rails)
+- **Language**: Ruby
+- **Database**: PostgreSQL
+- **License**: AGPL-3.0
+
+## Source
+
+| Resource | URL |
+|----------|-----|
+| **API Repository** | https://github.com/accentor/api |
+| **Web Frontend** | https://github.com/accentor/web |
+| **Android App** | https://github.com/accentor/android |
+| **Documentation** | https://accentor.tech |
+
+## Metadata Features
+
+- Albums can have **multiple artists** with different names per album/track
+- Albums can have **multiple labels**
+- Tracks can have **multiple genres**
+- Complete user control over metadata editing
+
+## Architecture
+
+```
+accentor/
+├── api/         # Rails API backend
+├── web/         # Vue.js frontend
+└── android/     # Android app
+```
+
+## Self-Hosting
+
+```bash
+# Clone and setup
+git clone https://github.com/accentor/api.git
+cd api
+bundle install
+rails db:setup
+
+# Run server (port 3000)
+puma -C config/puma.rb
+```
+
+Use nginx as reverse proxy:
+- Match `/api` and `/rails` paths → proxy to Puma
+- Serve web frontend on root
+
+## API Endpoints
+
+```bash
+GET /api/artists
+GET /api/artists/:id
+GET /api/albums
+GET /api/albums/:id
+GET /api/tracks
+GET /api/tracks/:id
+```
+
+## Notes
+
+- Designed for users who want precise metadata control
+- Build your own collection from CDs, Bandcamp, etc.
+- Sound quality you choose (not compressed by service)
+- Stream via web or Android app