fix(backend): log create_slab driver errors server-side

QUALITY-120 keeps the driver stack out of the owner-facing response, but the
bare except logged nothing, so a failing create was undiagnosable in the field.
Log the exception server-side while keeping the generic owner message.

backend/slabs.py

@@ -6,6 +6,7 @@ sets deleted_at; published slabs cannot be deleted.
 """
 
 import json
+import logging
 from datetime import datetime, timezone
 from typing import Any, Dict, List
 
@@ -15,6 +16,8 @@ from fastapi.responses import JSONResponse, Response
 from .db import get_connection
 from .thickness_band import load_thickness_terms, resolve_thickness_band_id
 
+logger = logging.getLogger("slab.backend")
+
 router = APIRouter(prefix="/api/v1/slabs")
 
 # Client-settable slab columns (explicit allowlist; never build SQL from client keys).
@@ -224,8 +227,10 @@ async def create_slab(payload: Dict[str, Any]) -> Dict[str, Any]:
     try:
         conn.execute(f"INSERT INTO slabs ({', '.join(cols)}) VALUES ({ph})", vals)
         conn.commit()
-    except Exception:
-        # QUALITY-120: never surface a driver stack to the owner.
+    except Exception as exc:
+        # QUALITY-120: never surface a driver stack to the owner, but log it
+        # server-side so a failing create is diagnosable.
+        logger.error("create_slab insert failed: %s", exc, exc_info=True)
         return _err("validation", "Could not create the slab. Check the fields and try again.", status=400)
     row = conn.execute("SELECT * FROM slabs WHERE id=?", (payload["id"],)).fetchone()
     return _row_to_slab(row)