fix(frontend): Add health check API endpoint for Docker monitoring

Resolve BUG-004 - frontend container unhealthy status.

Changes:
- Created /api/health endpoint using Next.js 15 App Router
- Supports GET and HEAD requests for health checks
- Returns JSON with status, timestamp, uptime, environment info
- Docker container now shows 'healthy' status

Fixes:
- Docker healthcheck endpoint missing (BUG-004)
- Container status showing 'unhealthy' despite working correctly

Testing:
- Verified endpoint returns 200 OK with health data
- Confirmed Docker container status changed to 'healthy'
- Health check interval: 30s, timeout: 10s, retries: 3

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/api/health/route.ts

@@ -0,0 +1,26 @@
+import { NextResponse } from 'next/server';
+
+/**
+ * Health check endpoint for Docker container monitoring
+ * Returns 200 OK with status information
+ */
+export async function GET() {
+  // Basic health check - always return healthy if the app is running
+  const healthData = {
+    status: 'healthy',
+    timestamp: new Date().toISOString(),
+    uptime: process.uptime(),
+    environment: process.env.NODE_ENV,
+    version: process.env.npm_package_version || 'unknown',
+  };
+
+  return NextResponse.json(healthData, { status: 200 });
+}
+
+/**
+ * Support HEAD requests for lightweight health checks
+ * Used by Docker healthcheck to minimize network traffic
+ */
+export async function HEAD() {
+  return new NextResponse(null, { status: 200 });
+}