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>

middleware/metrics.py

@@ -0,0 +1,85 @@
+"""
+Prometheus metrics middleware for FastAPI.
+
+Exposes /metrics endpoint in Prometheus text format.
+Tracks HTTP requests, translations, and file uploads.
+"""
+
+import time
+import logging
+from starlette.middleware.base import BaseHTTPMiddleware
+from starlette.requests import Request
+from starlette.responses import Response
+from prometheus_client import Counter, Histogram, generate_latest, CONTENT_TYPE_LSP
+
+logger = logging.getLogger(__name__)
+
+# ---- Metrics definitions ----
+
+http_requests_total = Counter(
+    "http_requests_total",
+    "Total HTTP requests",
+    ["method", "path", "status"],
+)
+
+translation_total = Counter(
+    "translation_total",
+    "Total translations processed",
+    ["provider", "file_type", "status"],
+)
+
+translation_duration_seconds = Histogram(
+    "translation_duration_seconds",
+    "Translation processing duration in seconds",
+    ["provider", "file_type"],
+    buckets=(0.5, 1, 2, 5, 10, 30, 60, 120, 300),
+)
+
+file_size_bytes = Histogram(
+    "file_size_bytes",
+    "Uploaded file size in bytes",
+    ["file_type"],
+    buckets=(100_000, 500_000, 1_000_000, 5_000_000, 10_000_000, 25_000_000, 50_000_000),
+)
+
+# Paths to skip from metrics (noisy health checks)
+_SKIP_PATHS = {"/health", "/ready", "/metrics", "/favicon.ico"}
+
+
+def record_translation(provider: str, file_type: str, duration: float, status: str = "success"):
+    translation_total.labels(provider=provider, file_type=file_type, status=status).inc()
+    translation_duration_seconds.labels(provider=provider, file_type=file_type).observe(duration)
+
+
+def record_file_size(file_type: str, size_bytes: int):
+    file_size_bytes.labels(file_type=file_type).observe(size_bytes)
+
+
+class PrometheusMiddleware(BaseHTTPMiddleware):
+    async def dispatch(self, request: Request, call_next):
+        if request.url.path in _SKIP_PATHS:
+            return await call_next(request)
+
+        start = time.time()
+        response: Response = await call_next(request)
+        duration = time.time() - start
+
+        path = request.url.path
+        # Group dynamic paths to avoid label explosion
+        if path.startswith("/api/v1/translations/"):
+            path = "/api/v1/translations/{id}"
+        elif path.startswith("/api/v1/download/"):
+            path = "/api/v1/download/{id}"
+
+        http_requests_total.labels(
+            method=request.method,
+            path=path,
+            status=str(response.status_code),
+        ).inc()
+
+        return response
+
+
+def get_metrics() -> Response:
+    body = generate_latest()
+    return Response(content=body, media_type=CONTENT_TYPE_LSP)