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/handlers/upload.rs

@@ -747,6 +747,30 @@ pub async fn get_preview(
     .await
 }
 
+/// Streaming access to an upload's big-screen **display** derivative (~2048px), used by the
+/// diashow. Gated identically to [`get_preview`]. 404s when the derivative doesn't exist yet
+/// (still compressing, or an old upload the backfill hasn't reached) — the diashow then falls
+/// back to the original.
+pub async fn get_display(
+    State(state): State<AppState>,
+    Path(upload_id): Path<Uuid>,
+) -> Result<axum::response::Response, AppError> {
+    let media = Upload::find_visible_media(&state.pool, upload_id)
+        .await?
+        .ok_or_else(|| AppError::NotFound("Upload nicht gefunden.".into()))?;
+    let rel = media
+        .display_path
+        .ok_or_else(|| AppError::NotFound("Anzeige nicht verfügbar.".into()))?;
+    let absolute = state.config.media_path.join(&rel);
+    stream_media_file(
+        &absolute,
+        "image/jpeg".to_string(),
+        "inline",
+        "private, max-age=300",
+    )
+    .await
+}
+
 /// Streaming access to an upload's **thumbnail** (video poster). Gated identically to
 /// [`get_preview`].
 pub async fn get_thumbnail(