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>

README.md

@@ -14,14 +14,43 @@ Upon first run it will generate thumbnails for all images and videos at `BASE_PA
 - **RAG-based Context Retrieval** - Semantic search over daily conversation summaries
 - **Automatic Daily Summaries** - LLM-generated summaries of daily conversations with embeddings
 
+## External Dependencies
+
+### ffmpeg (required)
+`ffmpeg` must be on `PATH`. It is used for:
+- **HLS video streaming** — transcoding/segmenting source videos into `.m3u8` + `.ts` playlists
+- **Video thumbnails** — extracting a frame at the 3-second mark
+- **Video preview clips** — short looping previews for the Video Wall
+- **HEIC / HEIF thumbnails** — decoding Apple's HEIC format (your ffmpeg build must include
+  `libheif`; most modern builds do)
+
+Builds used in development: the `gyan.dev` full build on Windows, and distro `ffmpeg`
+packages on Linux work fine. If HEIC thumbnails silently fail, check
+`ffmpeg -formats | grep heif` to confirm HEIF support.
+
+### RAW photo thumbnails (no extra dependency)
+RAW formats (ARW, NEF, CR2, CR3, DNG, RAF, ORF, RW2, PEF, SRW, TIFF) are thumbnailed
+by reading the embedded JPEG preview from the TIFF IFD1 using `kamadak-exif`. No
+external RAW decoder (libraw / dcraw) is required. Files without an embedded preview
+fall back to ffmpeg (works for most NEF files), and anything that still can't be
+decoded is marked with a `<thumb>.unsupported` sentinel in the thumbnail directory
+so we don't retry it every scan. Delete those sentinels to force retries after a
+tooling upgrade.
+
 ## Environment
 There are a handful of required environment variables to have the API run.
 They should be defined where the binary is located or above it in an `.env` file.
-You must have `ffmpeg` installed for streaming video and generating video thumbnails.
 
 - `DATABASE_URL` is a path or url to a database (currently only SQLite is tested)
 - `BASE_PATH` is the root from which you want to serve images and videos
-- `THUMBNAILS` is a path where generated thumbnails should be stored
+- `THUMBNAILS` is a path where generated thumbnails should be stored. Thumbnails
+  mirror the source tree under `BASE_PATH` and keep the source's original
+  extension (e.g. `foo.arw` or `bar.mp4`), though the file contents are always
+  JPEG bytes — browsers content-sniff. Files that can't be thumbnailed by the
+  `image` crate, ffmpeg, or an embedded RAW preview get a zero-byte
+  `<thumb_path>.unsupported` sentinel in this directory so subsequent scans
+  skip them. Delete the `*.unsupported` files to force retries (for example
+  after upgrading ffmpeg or adding libheif)
 - `VIDEO_PATH` is a path where HLS playlists and video parts should be stored
 - `GIFS_DIRECTORY` is a path where generated video GIF thumbnails should be stored
 - `BIND_URL` is the url and port to bind to (typically your own IP address)