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/harmony/README.md

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+# Harmony
+
+## Overview
+
+Music Metadata Aggregator and MusicBrainz Importer. Looks up releases from multiple providers, harmonizes the data into a common format, and supports intelligent merging and MusicBrainz seeding.
+
+## Key Features
+
+- **Providers**: MusicBrainz, Spotify, Deezer, Bandcamp, Beatport, iTunes, Tidal, KKBOX, Mora, Ototoy
+- **Lookup**: By GTIN (barcode), URL, or provider-specific ID
+- **Merging**: Intelligent algorithm to combine metadata from multiple sources
+- **Output**: Harmonized data representation, MusicBrainz release seeding
+- **License**: Not specified
+
+## Source
+
+| Resource | URL |
+|----------|-----|
+| **Repository** | https://github.com/kellnerd/harmony |
+| **Live Demo** | https://harmony.pulsewidth.org.uk |
+
+## Architecture
+
+Built with:
+- **Runtime**: Deno
+- **Framework**: Fresh (web framework)
+- **API**: REST
+
+Key components:
+- `providers/` - Provider implementations for each source
+- `lookup.ts` - Combined release lookup with parallel queries
+- `harmonizer/` - Data normalization and merging
+- `server/` - Web app and API routes
+
+## How It Works
+
+1. Accept GTIN, URL, or provider ID
+2. Query matching providers in parallel
+3. Convert each response to harmonized format
+4. Merge results using intelligent algorithm
+5. Optionally seed to MusicBrainz
+
+## Self-Hosting
+
+```bash
+# Requires Deno
+git clone https://github.com/kellnerd/harmony.git
+cd harmony
+deno task start
+```
+
+## Notes
+
+- Best multi-source aggregator with intelligent deduplication
+- Permalink support for cached snapshots
+- Automatic language/script detection
+- Active development (218 stars)