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

frontend/src/App.tsx

@@ -10,6 +10,7 @@ import { TimeEntriesPage } from "./pages/TimeEntriesPage";
 import { ClientsPage } from "./pages/ClientsPage";
 import { ProjectsPage } from "./pages/ProjectsPage";
 import { StatisticsPage } from "./pages/StatisticsPage";
+import { ApiKeysPage } from "./pages/ApiKeysPage";
 
 function App() {
   return (
@@ -33,6 +34,7 @@ function App() {
           <Route path="clients" element={<ClientsPage />} />
           <Route path="projects" element={<ProjectsPage />} />
           <Route path="statistics" element={<StatisticsPage />} />
+          <Route path="api-keys" element={<ApiKeysPage />} />
         </Route>
       </Routes>
     </AuthProvider>