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/musicmetalinker/README.md

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+# MusicMetaLinker
+
+## Overview
+
+Python library for entity linking and knowledge augmentation for music metadata. Links music tracks to external sources (MusicBrainz, AcousticBrainz, YouTube Music, Deezer) for metadata enrichment.
+
+## Key Features
+
+- **Purpose**: Entity linking across music databases
+- **Sources**: MusicBrainz, AcousticBrainz, YouTube Music, Deezer
+- **Matching**: Intelligent service selection based on available metadata
+- **License**: MIT
+
+## Source
+
+| Resource | URL |
+|----------|-----|
+| **Repository** | https://github.com/andreamust/MusicMetaLinker |
+| **PyPI** | https://pypi.org/project/MusicMetaLinker |
+
+## How It Works
+
+1. **Service Selection**: Evaluates available metadata, selects best external service
+2. **Information Retrieval**: Connects to service API, searches for best match
+3. **Filtering and Return**: Filters results, returns enriched metadata
+
+## Usage Example
+
+```python
+from musicmetalinker import MusicMetaLinker
+
+# Initialize with known metadata
+linker = MusicMetaLinker(
+    track_name="Bohemian Rhapsody",
+    artist_name="Queen",
+    album_name="A Night at the Opera"
+)
+
+# Get linked metadata
+track_name = linker.get_track()
+artist_name = linker.get_artist()
+album_name = linker.get_album()
+duration = linker.get_duration()
+isrc = linker.get_isrc()
+
+# Get external IDs
+links = {
+    'mbid': linker.get_mbid(),
+    'isrc': linker.get_isrc(),
+    'deezer_id': linker.get_deezer_id()
+}
+
+# Or query by single identifier
+linker = MusicMetaLinker(mbid="b10bbbfc-cf9e-42e0-be17-e2c3e1d2600d")
+linker = MusicMetaLinker(isrc="GBUM71029604")
+```
+
+## Installation
+
+```bash
+pip install MusicMetaLinker
+```
+
+## Notes
+
+- Best for enriching existing metadata with external links
+- Automatic service selection based on input
+- Can query by MBID, ISRC, or Deezer ID directly