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/models/upload.rs

@@ -46,6 +46,7 @@ pub struct VisibleMedia {
     pub original_path: String,
     pub preview_path: Option<String>,
     pub thumbnail_path: Option<String>,
+    pub display_path: Option<String>,
     pub mime_type: String,
 }
 
@@ -93,7 +94,7 @@ impl Upload {
         id: Uuid,
     ) -> Result<Option<VisibleMedia>, sqlx::Error> {
         sqlx::query_as::<_, VisibleMedia>(
-            "SELECT up.original_path, up.preview_path, up.thumbnail_path, up.mime_type
+            "SELECT up.original_path, up.preview_path, up.thumbnail_path, up.display_path, up.mime_type
              FROM upload up
              JOIN \"user\" u ON u.id = up.user_id
              WHERE up.id = $1 AND up.deleted_at IS NULL
@@ -134,6 +135,19 @@ impl Upload {
         Ok(())
     }
 
+    pub async fn set_display_path(
+        pool: &PgPool,
+        id: Uuid,
+        display_path: &str,
+    ) -> Result<(), sqlx::Error> {
+        sqlx::query("UPDATE upload SET display_path = $2 WHERE id = $1")
+            .bind(id)
+            .bind(display_path)
+            .execute(pool)
+            .await?;
+        Ok(())
+    }
+
     pub async fn set_thumbnail_path(
         pool: &PgPool,
         id: Uuid,