fix(scan): quiet startup scans & thumbnail RAW/HEIC

Three recurring issues on every full scan:

1. Video playlist scans re-enqueued every file only to reject it as
   AlreadyExists. Pre-filter in ScanDirectoryMessage and QueueVideosMessage
   so we skip videos whose .m3u8 already exists, and demote the leaked
   AlreadyExists log to debug.

2. image crate was built with only jpeg/png features, so webp/tiff/avif
   files logged "format not supported" every scan. Enable those features.

3. RAW (ARW/NEF/CR2/...) and HEIC thumbnails weren't generated, so the
   scan kept retrying them. Try the file's embedded JPEG preview via
   kamadak-exif first (fast, pure-Rust, works on Sony ARW where ffmpeg's
   TIFF decoder fails). Fall back to ffmpeg for HEIC/HEIF and RAWs with
   no preview. Anything still undecodable gets a <thumb>.unsupported
   sentinel so future scans skip it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/file_types.rs

@@ -3,9 +3,22 @@ use walkdir::DirEntry;
 
 /// Supported image file extensions
 pub const IMAGE_EXTENSIONS: &[&str] = &[
-    "jpg", "jpeg", "png", "webp", "tiff", "tif", "heif", "heic", "avif", "nef",
+    "jpg", "jpeg", "png", "webp", "tiff", "tif", "heif", "heic", "avif", "nef", "arw",
 ];
 
+/// Extensions the `image` crate cannot decode — we fall back to ffmpeg to
+/// extract an embedded preview or decode the frame.
+pub const FFMPEG_THUMBNAIL_EXTENSIONS: &[&str] = &["heif", "heic", "nef", "arw"];
+
+/// Returns true if thumbnail generation should go through ffmpeg instead of
+/// the `image` crate (RAW formats, HEIF/HEIC).
+pub fn needs_ffmpeg_thumbnail(path: &Path) -> bool {
+    match path.extension().and_then(|e| e.to_str()) {
+        Some(ext) => FFMPEG_THUMBNAIL_EXTENSIONS.contains(&ext.to_lowercase().as_str()),
+        None => false,
+    }
+}
+
 /// Supported video file extensions
 pub const VIDEO_EXTENSIONS: &[&str] = &["mp4", "mov", "avi", "mkv"];