feat: add MCP endpoint and API key management

- Add ApiKey Prisma model (SHA-256 hash, prefix, lastUsedAt) with migration
- Implement ApiKeyService (create, list, delete, verify)
- Extend requireAuth middleware to accept sk_-prefixed API keys alongside JWTs
- Add GET/POST /api-keys routes for creating and revoking keys
- Add stateless Streamable HTTP MCP server at POST/GET /mcp exposing all 20
  time-tracking tools (clients, projects, time entries, timer, statistics,
  client targets and corrections)
- Frontend: ApiKey types, apiKeys API module, useApiKeys hook
- Frontend: ApiKeysPage with key table, one-time raw-key reveal modal, and
  inline revoke confirmation
- Wire /api-keys route and add API Keys link to Management dropdown in Navbar

backend/package.json

@@ -10,6 +10,7 @@
     "db:seed": "tsx prisma/seed.ts"
   },
   "dependencies": {
+    "@modelcontextprotocol/sdk": "^1.27.1",
     "@prisma/client": "^6.19.2",
     "@quixo3/prisma-session-store": "^3.1.19",
     "cors": "^2.8.5",