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/acoustid/README.md

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+# AcoustID
+
+## Overview
+
+AcoustID is an open-source audio fingerprinting service. It identifies music tracks by their acoustic fingerprint and links them to MusicBrainz recordings.
+
+## Key Features
+
+- **Purpose**: Audio identification via acoustic fingerprinting
+- **Technology**: Chromaprint fingerprint generation
+- **Database**: Crowdsourced fingerprints linked to MusicBrainz
+- **License**: MIT (code), CC BY-SA 3.0 (data)
+
+## Source
+
+| Resource | URL |
+|----------|-----|
+| **Server Repository** | https://github.com/acoustid/acoustid-server |
+| **Index Repository** | https://github.com/acoustid/acoustid-index |
+| **Chromaprint Library** | https://github.com/acoustid/chromaprint |
+| **API Documentation** | https://acoustid.org/webservice |
+| **Website** | https://acoustid.org |
+
+## API Examples
+
+```bash
+# Lookup by fingerprint
+GET /v2/lookup?client=YOUR_API_KEY&meta=recordings&fingerprint={fp}&duration={dur}
+
+# Submit new fingerprint
+POST /v2/submit
+```
+
+## Chromaprint CLI
+
+```bash
+# Generate fingerprint from audio file
+fpcalc song.mp3
+# Returns: FINGERPRINT=... DURATION=...
+```
+
+## Self-Hosting
+
+The acoustid-index v2 is written in Zig for performance:
+
+```bash
+git clone https://github.com/acoustid/acoustid-index.git
+# Follow build instructions in README
+```
+
+## Notes
+
+- Used by: Beets, Picard, Kid3, MusicBrainz ecosystem
+- Free API for audio fingerprint matching
+- Identify unknown files → get MusicBrainz metadata