image: add on-demand size=large preview tier (~2048px JPEG q85)

Adds a third PhotoSize between Thumb (200px) and Full (original). The
viewer placeholder and map callout previously upscaled a 200px thumb
into a full-screen / full-width view, which looked visibly blocky on
3× devices. The new tier is generated on-demand, disk-cached, and
served via the existing /image endpoint.

Storage layout mirrors the Thumb branch's lookup chain:
  1. hash-keyed: <thumbs>/_large/<hash[..2]>/<hash>.jpg (shared across
     libraries when content_hash is known)
  2. library-scoped legacy: <thumbs>/_large/<lib_id>/<rel_path>

Generation pipeline mirrors generate_image_thumbnail:
  - RAW: decode the embedded JPEG preview, apply EXIF orientation,
         resize to 2048-long-edge, encode JPEG q85
  - HEIC/HEIF: ffmpeg with scale + q:v 5 (≈ q85)
  - everything else: image crate decode + thumbnail() + JpegEncoder
Never upscales — sources below the 2048 cap re-encode at native size.

Handler offloads decode/resize to web::block to keep the actix worker
free (a 24MP source takes 100–500ms). Writes via tempfile+rename so
concurrent readers can't observe a half-written JPEG. On any
generation failure, falls through to the Full branch (which itself
serves the RAW embedded preview for unrenderable RAW containers).

Video requests for size=large fall back to the existing thumb pipeline
since there's no useful 2048px video tier.

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

src/content_hash.rs

@@ -50,6 +50,18 @@ pub fn thumbnail_path(thumbs_dir: &Path, hash: &str) -> PathBuf {
     thumbs_dir.join(shard).join(format!("{}.jpg", hash))
 }
 
+/// Hash-keyed large-preview path: `<thumbs_dir>/_large/<hash[..2]>/<hash>.jpg`.
+/// Kept under the same root as 200px thumbs so deployments don't need a
+/// second env var, but namespaced under `_large/` so the existing 200px
+/// shards don't collide with the larger derivative.
+pub fn large_preview_path(thumbs_dir: &Path, hash: &str) -> PathBuf {
+    let shard = shard_prefix(hash);
+    thumbs_dir
+        .join("_large")
+        .join(shard)
+        .join(format!("{}.jpg", hash))
+}
+
 /// Hash-keyed HLS output directory: `<video_dir>/<hash[..2]>/<hash>/`.
 /// The playlist lives at `playlist.m3u8` inside this directory and its
 /// segments are co-located so HLS relative references Just Work. See
@@ -120,6 +132,9 @@ mod tests {
         let p = thumbnail_path(thumbs, "abcdef0123");
         assert_eq!(p, PathBuf::from("/tmp/thumbs/ab/abcdef0123.jpg"));
 
+        let l = large_preview_path(thumbs, "abcdef0123");
+        assert_eq!(l, PathBuf::from("/tmp/thumbs/_large/ab/abcdef0123.jpg"));
+
         let video = Path::new("/tmp/video");
         let d = hls_dir(video, "1234deadbeef");
         assert_eq!(d, PathBuf::from("/tmp/video/12/1234deadbeef"));