fix: ffmpeg fallback for HEIC files that libheif rejects

iPhone photos with depth maps or gain maps have too many auxiliary
image references for libheif 1.17, causing pillow-heif to throw
"Too many auxiliary image references". process_heic_image() now
falls back to ffmpeg when pillow-heif fails — ffmpeg's own HEIC
decoder handles these files without issue. Fixes 27/35 HEIC photos
that were stuck in failed state.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/tasks/thumbs.py

@@ -102,17 +102,36 @@ def extract_raw_preview(filepath: str) -> Optional[Image.Image]:
     return None
 
 def process_heic_image(filepath: str) -> Image.Image:
-    """Process HEIC/HEIF image formats"""
+    """Process HEIC/HEIF image formats.
+
+    Tries pillow-heif first (fast, native). Falls back to ffmpeg for
+    files that libheif rejects — e.g. iPhone photos with too many
+    auxiliary image references (depth maps, gain maps).
+    """
     try:
-        # Use pillow-heif to open the image
         img = Image.open(filepath)
-        # Convert to RGB if needed
         if img.mode != 'RGB':
             img = img.convert('RGB')
         return img
     except Exception as e:
-        logger.error(f"Error processing HEIC file {filepath}: {e}")
-        raise
+        logger.warning(f"pillow-heif failed for {filepath}: {e} — trying ffmpeg")
+
+    # ffmpeg fallback: decode HEIC to PNG in memory.
+    import subprocess
+    from io import BytesIO
+    try:
+        result = subprocess.run(
+            ['ffmpeg', '-i', filepath, '-frames:v', '1',
+             '-f', 'image2pipe', '-vcodec', 'png', 'pipe:1'],
+            capture_output=True, timeout=30, stdin=subprocess.DEVNULL,
+        )
+        if result.returncode == 0 and result.stdout:
+            img = Image.open(BytesIO(result.stdout)).convert('RGB')
+            return img
+        logger.error(f"ffmpeg HEIC decode failed for {filepath}: {result.stderr.decode()[-200:]}")
+    except Exception as e2:
+        logger.error(f"ffmpeg fallback failed for {filepath}: {e2}")
+    raise RuntimeError(f"Cannot decode HEIC: {filepath}")
 
 def process_video_thumbnail(filepath: str) -> Image.Image:
     """Extract a still frame from a video file as a PIL Image."""