feat: Plugin marketplace / community registry (#66)

[deucebucket]

Parent: #66 Phase 5 - Custom Layers & Plugin System

What

In-app plugin browser backed by a GitHub-hosted JSON registry. Users discover and one-click install community plugins.

Registry (hosted at github.com/deucebucket/library-manager-plugins)

{
  "schema_version": 1,
  "plugins": [{
    "id": "hardcover-lookup",
    "name": "Hardcover.app Lookup",
    "description": "Look up book metadata from Hardcover.app",
    "author": "contributor_name",
    "version": "1.0.0",
    "type": "layer",
    "download_url": "https://github.com/.../releases/.../hardcover-lookup.zip",
    "verified": true,
    "tags": ["api", "metadata"]
  }]
}

Implementation

• Create deucebucket/library-manager-plugins repo with registry JSON
• GET /api/plugins/registry - fetch + cache registry (24h TTL)
• Browse UI in Settings → Plugins tab (card grid with search/filter)
• One-click install: download zip → extract to /data/plugins/<id>/
• Verified badge vs "unverified - runs arbitrary code" warning
• Update/uninstall from UI
• Plugin submission via PR to registry repo

Depends on

• Drop-in plugin system (needs the loader to exist first)

Part of

#66 Phase 5, Tier 5 (build last - needs community)