fix(vision): file non immagine e grafo malformato non escono più come 500 (#15)

Worker: Image.open().convert() in try/except (422 "image is not
decodable"); il grafo deve essere un dict (422 altrimenti); run_graph
distingue costruzione (GraphBuildError -> 422) da esecuzione (500 con
motivo). Router: verifica dei magic bytes (PNG/JPEG/BMP/TIFF/WebP/GIF)
prima di chiamare il worker.

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

src/vision_worker/main.py

@@ -10,9 +10,9 @@ import json
 
 import numpy as np
 from fastapi import FastAPI, File, Form, HTTPException, UploadFile, status
-from PIL import Image
+from PIL import Image, UnidentifiedImageError
 
-from src.vision.runner import engine_version, run_graph
+from src.vision.runner import GraphBuildError, engine_version, run_graph
 
 app = FastAPI(title="TieMeasureFlow Vision Worker", version="0.1.0")
 
@@ -35,17 +35,36 @@ async def run(
             detail=f"graph is not valid JSON: {exc}",
         ) from exc
 
-    frame = np.array(Image.open(image.file).convert("L"))
+    if not isinstance(parsed_graph, dict):
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_CONTENT,
+            detail=f"graph must be a JSON object, got {type(parsed_graph).__name__}",
+        )
+
+    try:
+        frame = np.array(Image.open(image.file).convert("L"))
+    except (UnidentifiedImageError, OSError) as exc:
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_CONTENT,
+            detail=f"image is not decodable: {exc}",
+        ) from exc
 
     try:
         outcome = run_graph(frame, parsed_graph)
-    except ValueError as exc:
-        # from_dict refuses a schema version it does not handle; that is a bad
-        # request, not a server fault.
+    except GraphBuildError as exc:
+        # The graph itself is bad (schema version, a tool missing "kind", ...) -
+        # a bad request, not a server fault.
         raise HTTPException(
             status_code=status.HTTP_422_UNPROCESSABLE_CONTENT,
             detail=str(exc),
         ) from exc
+    except Exception as exc:
+        # The graph built fine; something failed while actually running it -
+        # an engine fault, not the caller's. Still named, not a naked 500.
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail=f"vision run failed: {type(exc).__name__}: {exc}",
+        ) from exc
 
     return {
         # `outcome.outputs` never carries NaN/inf/bool (see runner.run_graph):