feat: merge cartsnitch/api into api/ subdirectory

Consolidate API gateway service into monorepo.
Squashed from https://github.com/cartsnitch/api main (89bacb1).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/cartsnitch_api/routes/health.py

@@ -0,0 +1,20 @@
+"""Health check and error metrics endpoints."""
+
+from fastapi import APIRouter, Depends
+
+from cartsnitch_api.auth.dependencies import verify_service_key
+from cartsnitch_api.middleware.error_handler import get_error_monitor
+
+router = APIRouter(tags=["health"])
+
+
+@router.get("/health")
+async def health():
+    return {"status": "ok"}
+
+
+@router.get("/internal/error-stats", dependencies=[Depends(verify_service_key)])
+async def error_stats():
+    """Error monitoring stats — internal only (requires X-Service-Key)."""
+    monitor = get_error_monitor()
+    return monitor.get_stats()