5.5: Delete all legacy code

[MMTE]

Files to delete

• apps/api/src/lib/plugins/ (internal PluginManager, BasePlugin, sub-plugins)
• apps/web/src/lib/plugins/ (frontend PluginManager, PluginLoader, PluginRegistry, RouteManager, UISlotManager)
• All remaining apps/api/src/lib/ directories
• All remaining apps/api/src/routers/ tRPC routers
• apps/api/src/trpc/ directory
• apps/web/src/api/trpc.ts, providers/TRPCProvider.tsx
• apps/web/src/lib/billing/ProrationCalculator.ts (business logic in frontend)
• apps/web/src/lib/events/EventEmitter.ts
• apps/web/src/pages/LandingPage.tsx (30KB marketing page)
• Remove tRPC deps from all package.json
• Remove Express deps from apps/api/package.json
• Clean up README.md

Depends on all modules migrated and frontend shell working.