Portraits: set correct Content-Type for cloud images (NIM returns JPEG)

Sniff the image magic bytes so the response Content-Type matches the actual
format (FLUX/NIM returns JPEG, local Z-Image returns PNG) — keeps the cached
blob's type correct on the client.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app.py

@@ -508,6 +508,16 @@ def _hf_portrait(prompt, model="black-forest-labs/FLUX.1-schnell"):
     return data, None
 
 
+def _img_mime(data):
+    if data[:3] == b"\xff\xd8\xff":
+        return "image/jpeg"  # NIM/FLUX returns JPEG
+    if data[:8] == b"\x89PNG\r\n\x1a\n":
+        return "image/png"   # local Z-Image
+    if data[:4] == b"RIFF" and data[8:12] == b"WEBP":
+        return "image/webp"
+    return "image/png"
+
+
 @fastapi_app.post("/portrait")
 async def portrait(request: Request):
     body = await request.json()
@@ -535,7 +545,7 @@ async def portrait(request: Request):
         return Response("no image provider (set NVIDIA_NIM_API_KEY / HF_TOKEN, or TINY_IMAGE_MODE=local)", status_code=503)
     if err:
         return Response(err, status_code=502)
-    return Response(png, media_type="image/png", headers={"Cache-Control": "no-store"})
+    return Response(png, media_type=_img_mime(png), headers={"Cache-Control": "no-store"})
 
 
 @fastapi_app.get("/persona/status")