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/api/apiKeys.ts

@@ -0,0 +1,18 @@
+import apiClient from './client';
+import type { ApiKey, CreatedApiKey, CreateApiKeyInput } from '@/types';
+
+export const apiKeysApi = {
+  getAll: async (): Promise<ApiKey[]> => {
+    const { data } = await apiClient.get<ApiKey[]>('/api-keys');
+    return data;
+  },
+
+  create: async (input: CreateApiKeyInput): Promise<CreatedApiKey> => {
+    const { data } = await apiClient.post<CreatedApiKey>('/api-keys', input);
+    return data;
+  },
+
+  delete: async (id: string): Promise<void> => {
+    await apiClient.delete(`/api-keys/${id}`);
+  },
+};