feat: add Prometheus metrics + fix CI/CD health check port

- Add prometheus-client dependency
- Create middleware/metrics.py with PrometheusMiddleware
- Expose /metrics endpoint in Prometheus text format
- Track http_requests_total, translation_total, translation_duration_seconds,
  file_size_bytes
- Instrument translate routes with record_translation() and record_file_size()
- Fix deploy.yml health check: localhost:8000 -> localhost:8001 (Portainer conflict)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

main.py

@@ -56,6 +56,7 @@ from middleware.security import (
     RequestLoggingMiddleware,
 )
 from middleware.error_handler import ErrorHandlingMiddleware, format_error_response
+from middleware.metrics import PrometheusMiddleware, get_metrics
 from middleware.cleanup import (
     MemoryMonitor,
     HealthChecker,
@@ -359,6 +360,7 @@ app = FastAPI(
 app.openapi = custom_openapi
 
 app.add_middleware(ErrorHandlingMiddleware)
+app.add_middleware(PrometheusMiddleware)
 app.add_middleware(RequestLoggingMiddleware, log_body=False)
 app.add_middleware(
     SecurityHeadersMiddleware,
@@ -574,6 +576,12 @@ async def root():
     }
 
 
+@app.get("/metrics", tags=["Health"])
+async def metrics_endpoint():
+    """Prometheus metrics endpoint"""
+    return get_metrics()
+
+
 @app.get("/health", tags=["Health"])
 async def health_check():
     """Health check endpoint with detailed system status (Kubernetes liveness probe)"""