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/data/mod.rs

@@ -194,6 +194,7 @@ pub enum MediaType {
 #[serde(rename_all = "lowercase")]
 pub enum PhotoSize {
     Full,
+    Large,
     Thumb,
 }