Add MODE=pcb layout exam job with AI and deterministic checks.

Parallel pcb_status pipeline: parse board, classify domains/groups,
inventory traces, PE-LAY/PLC/SI/DRT checks, per-IC datasheet review.
Findings merge into the report UI. No auto-place or pcbnew write-back.

backend/pipeline_worker.py

@@ -104,8 +104,13 @@ async def _run() -> None:
     elif mode == "placement":
         from backend.services import placement_pipeline as placement_svc
         await placement_svc.run_placement_pipeline(storage, user_id, project_id)
+    elif mode == "pcb":
+        from backend.services import pcb_pipeline as pcb_svc
+        await pcb_svc.run_pcb_pipeline(storage, user_id, project_id)
     else:
-        raise SystemExit(f"unknown MODE={mode!r}; expected 'run', 'regen', or 'placement'")
+        raise SystemExit(
+            f"unknown MODE={mode!r}; expected 'run', 'regen', 'placement', or 'pcb'"
+        )
 
 
 def main() -> None: