[FEATURE] Create Plugin Marketplace/Registry

[JoshuaAFerguson]

Create a plugin marketplace for discovering and installing plugins.

API Endpoints:

• GET /api/v1/plugins/marketplace - List available plugins
• POST /api/v1/plugins/install - Install plugin
• DELETE /api/v1/plugins/{id} - Uninstall plugin

Plugin Metadata:

• Name, version, author
• Description, category
• Downloads, rating
• Verified badge

Acceptance Criteria:

• Marketplace API works
• UI for browsing plugins
• Install/uninstall works
• Ratings/reviews system

Files: api/internal/plugins/marketplace.go (NEW)