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/musicbrainz-server/README.md

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+# MusicBrainz Server
+
+## Overview
+
+MusicBrainz is the canonical open-source music encyclopedia. It's a community-maintained database that collects music metadata and makes it available to the public via a REST API.
+
+## Key Features
+
+- **Coverage**: 100M+ tracks, comprehensive artist/release/recording/work metadata
+- **Data**: Relationships, genres, tags, ratings, ISRCs, barcodes
+- **API**: REST (XML/JSON)
+- **License**: GPL-2.0 (code), CC0/CC BY-NC-SA (data)
+
+## Source
+
+| Resource | URL |
+|----------|-----|
+| **Repository** | https://github.com/metabrainz/musicbrainz-server |
+| **Docker Setup** | https://github.com/metabrainz/musicbrainz-docker |
+| **API Documentation** | https://musicbrainz.org/doc/MusicBrainz_API |
+| **Website** | https://musicbrainz.org |
+
+## API Examples
+
+```bash
+# Lookup artist by MBID
+GET /ws/2/artist/{mbid}?fmt=json
+
+# Search releases
+GET /ws/2/release?query=artist:nirvana&fmt=json
+
+# Lookup by ISRC
+GET /ws/2/recording?isrc=USEE10001993
+```
+
+## Self-Hosting
+
+Requires PostgreSQL + Solr. Use the official Docker setup:
+
+```bash
+git clone https://github.com/metabrainz/musicbrainz-docker.git
+cd musicbrainz-docker
+docker-compose up
+```
+
+## Notes
+
+- Foundation for most other music metadata projects
+- Replication support to keep local mirror up-to-date
+- MetaBrainz Foundation (non-profit) maintains it