Fix 16:9 output + MIME detection for Image-to-Image

- Pillow installiert (fehlte → try/except lief ins Leere)
- _detect_image_mime(): Magic-Bytes-Erkennung (JPEG/PNG/WebP/HEIC)
- _gemini_generate_image(): kein hartes 'image/jpeg' mehr
- PIL-Resize 1920×1080 greift jetzt korrekt

article_server.py

@@ -87,6 +87,21 @@ def _get_gemini_key() -> str:
         return ""
 
 
+def _detect_image_mime(image_bytes: bytes) -> str:
+    """MIME-Type anhand Magic-Bytes erkennen."""
+    if len(image_bytes) < 8:
+        return "image/jpeg"  # Fallback
+    if image_bytes[:4] == b"\x89PNG":
+        return "image/png"
+    if image_bytes[:2] == b"\xff\xd8":
+        return "image/jpeg"
+    if image_bytes[:4] == b"RIFF" and image_bytes[8:12] == b"WEBP":
+        return "image/webp"
+    if image_bytes[:4] in (b"ftyp", b"heic", b"heix", b"hevc", b"hevx"):
+        return "image/heic"
+    return "image/jpeg"  # Fallback
+
+
 def _gemini_generate_image(
     prompt: str,
     image_base64: str | None,
@@ -104,15 +119,20 @@ def _gemini_generate_image(
     Returns:
         dict mit "image_base64" (PNG) oder "error".
     """
+    import base64 as b64
+    
     key = _get_gemini_key()
     if not key:
         return {"error": "Kein Gemini API-Key"}
 
     parts = [{"text": f"{prompt}\n\nAspect ratio: {aspect_ratio}."}]
     if image_base64:
+        # MIME-Type aus den tatsächlichen Bilddaten erkennen
+        raw_bytes = b64.b64decode(image_base64)
+        mime_type = _detect_image_mime(raw_bytes)
         parts.append({
             "inlineData": {
-                "mimeType": "image/jpeg",
+                "mimeType": mime_type,
                 "data": image_base64
             }
         })