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/feed.rs

@@ -27,6 +27,8 @@ pub struct FeedUpload {
     pub uploader_name: String,
     pub preview_url: Option<String>,
     pub thumbnail_url: Option<String>,
+    /// Big-screen (~2048px) variant for the diashow. Absent until the derivative exists.
+    pub display_url: Option<String>,
     pub mime_type: String,
     pub caption: Option<String>,
     pub like_count: i64,
@@ -48,6 +50,7 @@ struct FeedRow {
     uploader_name: String,
     preview_path: Option<String>,
     thumbnail_path: Option<String>,
+    display_path: Option<String>,
     mime_type: String,
     caption: Option<String>,
     like_count: i64,
@@ -94,7 +97,8 @@ pub async fn feed(
         let tag = hashtag.trim().trim_start_matches('#').to_lowercase();
         sqlx::query_as::<_, FeedRow>(
             "SELECT v.id, v.user_id, v.uploader_name, v.preview_path, v.thumbnail_path,
-                    v.mime_type, v.caption, v.like_count, v.comment_count, v.created_at
+                    v.display_path, v.mime_type, v.caption, v.like_count, v.comment_count,
+                    v.created_at
              FROM v_feed v
              JOIN upload_hashtag uh ON uh.upload_id = v.id
              JOIN hashtag h ON h.id = uh.hashtag_id AND h.tag = $1
@@ -113,7 +117,7 @@ pub async fn feed(
     } else {
         sqlx::query_as::<_, FeedRow>(
             "SELECT id, user_id, uploader_name, preview_path, thumbnail_path,
-                    mime_type, caption, like_count, comment_count, created_at
+                    display_path, mime_type, caption, like_count, comment_count, created_at
              FROM v_feed
              WHERE event_id = $1
                AND ($2::timestamptz IS NULL OR (created_at, id) < ($2, $3))
@@ -154,6 +158,10 @@ pub async fn feed(
                 .thumbnail_path
                 .as_ref()
                 .map(|_| format!("/api/v1/upload/{}/thumbnail", r.id));
+            let display_url = r
+                .display_path
+                .as_ref()
+                .map(|_| format!("/api/v1/upload/{}/display", r.id));
             FeedUpload {
                 liked_by_me: liked_set.contains(&r.id),
                 id: r.id,
@@ -161,6 +169,7 @@ pub async fn feed(
                 uploader_name: r.uploader_name,
                 preview_url,
                 thumbnail_url,
+                display_url,
                 mime_type: r.mime_type,
                 caption: r.caption,
                 like_count: r.like_count,
@@ -247,7 +256,7 @@ pub async fn feed_delta(
     // response's `server_time`, so this doesn't re-fetch on every subsequent delta.
     let rows = sqlx::query_as::<_, FeedRow>(
         "SELECT id, user_id, uploader_name, preview_path, thumbnail_path,
-                mime_type, caption, like_count, comment_count, created_at
+                display_path, mime_type, caption, like_count, comment_count, created_at
          FROM v_feed
          WHERE event_id = $1 AND created_at >= $2
          ORDER BY created_at DESC, id DESC
@@ -304,6 +313,10 @@ pub async fn feed_delta(
                 .thumbnail_path
                 .as_ref()
                 .map(|_| format!("/api/v1/upload/{}/thumbnail", r.id)),
+            display_url: r
+                .display_path
+                .as_ref()
+                .map(|_| format!("/api/v1/upload/{}/display", r.id)),
             mime_type: r.mime_type,
             caption: r.caption,
             like_count: r.like_count,

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(