feat(plugins): implement plugin loader and integration points

[sasagar]

Summary

This PR implements Phase 2 of the plugin system: Plugin Loader and Integration Points.

Depends on: #134 (Phase 1 - Event Bus)

Changes

• PluginLoader class (packages/backend/src/plugins/loader.ts)

  • Discovers and loads plugins from filesystem
  • Handles plugin lifecycle (onLoad/onUnload)
  • Dependency resolution with topological sort
  • Version compatibility checking
  • Enable/disable functionality

• Plugin Configuration Storage

  • InMemoryPluginConfigStorage for development/testing
  • DatabasePluginConfigStorage for production (persistent)
  • New plugin_configs table in database schema

• Plugin System Initialization (packages/backend/src/plugins/init.ts)

  • Integrates with app startup
  • Registers plugin routes under /api/x/{pluginId}/
  • Registers plugin middleware
  • Provides ActivityPub handler registration

• App Integration (packages/backend/src/index.ts)

  • Plugin system initialization on startup
  • Graceful shutdown with plugin unloading
  • Environment variable configuration (PLUGINS_ENABLED, PLUGINS_DIR)

Plugin Directory Structure

Plugins are loaded from the ./plugins directory (configurable via PLUGINS_DIR):

plugins/
├── my-plugin/
│   ├── plugin.json    # or package.json
│   └── index.js       # Plugin entry point
└── another-plugin/
    ├── plugin.json
    └── index.js

Environment Variables

• PLUGINS_ENABLED: Set to "false" to disable plugin system (default: enabled)
• PLUGINS_DIR: Plugin directory path (default: "./plugins")
• INSTANCE_NAME: Instance name passed to plugin context

Test plan

• All 26 PluginLoader unit tests passing
• All 1012 backend tests passing
• TypeScript type checking passes
• Manual testing with sample plugin

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