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/exif.rs

@@ -1,5 +1,5 @@
 use std::fs::File;
-use std::io::BufReader;
+use std::io::{BufReader, Read, Seek, SeekFrom};
 use std::path::Path;
 
 use anyhow::{Result, anyhow};
@@ -25,6 +25,60 @@ pub struct ExifData {
     pub date_taken: Option<i64>,
 }
 
+/// TIFF-based RAW formats where `JPEGInterchangeFormat` offsets are
+/// absolute file offsets (the file itself is a TIFF container).
+fn is_tiff_raw(path: &Path) -> bool {
+    matches!(
+        path.extension()
+            .and_then(|e| e.to_str())
+            .map(|s| s.to_lowercase())
+            .as_deref(),
+        Some(
+            "tiff" | "tif" | "nef" | "cr2" | "arw" | "dng" | "raf" | "orf" | "rw2" | "pef" | "srw"
+        )
+    )
+}
+
+/// Returns the bytes of the embedded JPEG thumbnail in a TIFF-based RAW or
+/// TIFF file. Used to thumbnail formats whose RAW pixel data can't be decoded
+/// by our normal tools (e.g. Sony ARW). Returns `None` if no preview is
+/// present, the file isn't a TIFF container, or the data doesn't look like
+/// a valid JPEG.
+pub fn extract_embedded_jpeg_preview(path: &Path) -> Option<Vec<u8>> {
+    if !is_tiff_raw(path) {
+        return None;
+    }
+
+    let file = File::open(path).ok()?;
+    let mut bufreader = BufReader::new(file);
+    let exif = Reader::new().read_from_container(&mut bufreader).ok()?;
+
+    let offset = exif
+        .get_field(Tag::JPEGInterchangeFormat, In::THUMBNAIL)?
+        .value
+        .get_uint(0)?;
+    let length = exif
+        .get_field(Tag::JPEGInterchangeFormatLength, In::THUMBNAIL)?
+        .value
+        .get_uint(0)?;
+    if length == 0 {
+        return None;
+    }
+
+    let mut file = File::open(path).ok()?;
+    file.seek(SeekFrom::Start(offset as u64)).ok()?;
+    let mut buf = vec![0u8; length as usize];
+    file.read_exact(&mut buf).ok()?;
+
+    // JPEG SOI marker sanity check — MakerNote offsets sometimes point at
+    // TIFF-wrapped previews or other non-JPEG data.
+    if buf.len() < 2 || buf[0] != 0xFF || buf[1] != 0xD8 {
+        return None;
+    }
+
+    Some(buf)
+}
+
 pub fn supports_exif(path: &Path) -> bool {
     if let Some(ext) = path.extension() {
         let ext_lower = ext.to_string_lossy().to_lowercase();