feat(diashow): guarantee all eligible photos shown + 2048px display derivative

Diashow completeness rewrite so every eligible upload is shown regardless of
bursts, disconnects, or library size:

- queue.ts: SlideQueue with live/shuffle queues, allKnown map, recentlyShown
  ring; merge(dedup, live-first), remove/removeByUser (prunes recentlyShown),
  knownIds for reconcile-eviction. Adds queue.test.ts (burst/completeness/race).
- diashow/+page.svelte: reconcile (full paginate + evict, pre-scan snapshot to
  spare concurrent uploads) on mount/reconnect/periodic; catchUpNew paginate-
  until-known for bursts with debounced maxWait; hard-cut removals; decode
  timeout + candidate fallback + bounded skip so a broken image never stalls.

New ~2048px "display" derivative for big-screen sharpness, decoupled from the
data-saver preview (800px) used on phones:

- migration 016: upload.display_path + v_feed rebuilt (DROP+CREATE, not REPLACE,
  to slot the column beside preview/thumbnail).
- compression: generate_image_derivatives emits preview+display (downscale-only
  guard, no upscaling); backfill_missing_display regenerates on startup (safe:
  logs on error, never soft-deletes).
- upload.rs/main.rs: GET /upload/{id}/display (mirrors preview auth/cache),
  /media/displays direct-serve blocked.
- feed.rs + types.ts: display_url in feed/delta DTOs.
- diashow candidate chain: display -> original -> preview.

Verified on the running stack: migration applied, 10/10 existing images
backfilled (2048px cap honoured, small images not upscaled), /display serves
200, /feed returns display_url, diashow cycles.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/main.rs

@@ -45,6 +45,11 @@ async fn main() -> Result<()> {
 
     let state = AppState::new(pool.clone(), config.clone());
 
+    // Backfill the big-screen display derivative for image uploads processed before it
+    // existed (v0.17.x). Fire-and-forget behind the compression semaphore; each upload
+    // falls back to its original in the diashow until its display is generated.
+    state.compression.backfill_missing_display().await;
+
     // Re-spawn exports for events that were released but whose keepsake never finished
     // (crash mid-export). Needs the media/export paths + SSE sender, so it runs here
     // rather than inside `startup_recovery`. Fire-and-forget: the workers run in the
@@ -106,6 +111,10 @@ async fn main() -> Result<()> {
             "/api/v1/upload/{id}/preview",
             get(handlers::upload::get_preview),
         )
+        .route(
+            "/api/v1/upload/{id}/display",
+            get(handlers::upload::get_display),
+        )
         .route(
             "/api/v1/upload/{id}/thumbnail",
             get(handlers::upload::get_thumbnail),
@@ -241,6 +250,10 @@ async fn main() -> Result<()> {
             "/media/previews",
             get(|| async { axum::http::StatusCode::NOT_FOUND }),
         )
+        .nest_service(
+            "/media/displays",
+            get(|| async { axum::http::StatusCode::NOT_FOUND }),
+        )
         .nest_service(
             "/media/thumbnails",
             get(|| async { axum::http::StatusCode::NOT_FOUND }),