fix(vision): worker down o in timeout non esce più come 500 nudo (#14)

_call_worker distingue httpx.TimeoutException (504) dal resto dei
TransportError/HTTPError (502, RemoteProtocolError incluso). Un 422 del
worker si propaga come 422 con il motivo del worker, non più 502 generico.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/backend/services/vision_service.py

@@ -26,12 +26,14 @@ async def _call_worker(image_bytes: bytes, graph: dict) -> dict:
 
     The worker's fault must not become the API's fault (spec's edge-case
     table: "il worker isola il guasto, l'API resta in piedi, il task
-    riporta l'errore"). A stopped worker raises `httpx.ConnectError`; a job
-    past the timeout raises `httpx.ReadTimeout` - both would otherwise
-    propagate past this function and surface to the operator as a naked
-    500. The response shape is validated here too: a proxy answering 200
-    with something that is not the worker's JSON contract must not turn
-    into a bare KeyError further down.
+    riporta l'errore"). httpx.TimeoutException (ConnectTimeout, ReadTimeout,
+    WriteTimeout, PoolTimeout) becomes 504; every other transport failure
+    (ConnectError, ReadError, RemoteProtocolError when the worker dies
+    mid-response, ...) becomes 502 - both would otherwise propagate past
+    this function and surface to the operator as a naked 500. The response
+    shape is validated here too: a proxy answering 200 with something that
+    is not the worker's JSON contract must not turn into a bare KeyError
+    further down.
     """
     try:
         async with httpx.AsyncClient(timeout=120.0) as client:
@@ -40,16 +42,28 @@ async def _call_worker(image_bytes: bytes, graph: dict) -> dict:
                 files={"image": ("frame.png", image_bytes, "image/png")},
                 data={"graph": json.dumps(graph)},
             )
-    except httpx.ConnectError as exc:
-        raise HTTPException(
-            status_code=status.HTTP_502_BAD_GATEWAY,
-            detail=f"the vision worker is unreachable: {exc}",
-        ) from exc
-    except httpx.ReadTimeout as exc:
+    except httpx.TimeoutException as exc:
         raise HTTPException(
             status_code=status.HTTP_504_GATEWAY_TIMEOUT,
             detail=f"the vision worker did not answer in time: {exc}",
         ) from exc
+    except (httpx.TransportError, httpx.HTTPError) as exc:
+        raise HTTPException(
+            status_code=status.HTTP_502_BAD_GATEWAY,
+            detail=f"the vision worker is unreachable: {exc}",
+        ) from exc
+
+    if response.status_code == status.HTTP_422_UNPROCESSABLE_CONTENT:
+        # The worker already diagnosed a bad request (undecodable image,
+        # malformed graph) - forward its reason instead of flattening it
+        # into a generic 502.
+        try:
+            detail = response.json().get("detail", response.text)
+        except ValueError:
+            detail = response.text
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_CONTENT, detail=detail,
+        )
 
     if response.status_code != 200:
         raise HTTPException(