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/migrations/016_display_derivative.down.sql

@@ -0,0 +1,28 @@
+-- Drop the view (frees the column dependency), remove the column, then restore the
+-- pre-016 view definition (matches migration 011).
+DROP VIEW IF EXISTS v_feed;
+ALTER TABLE upload DROP COLUMN display_path;
+
+CREATE VIEW v_feed AS
+SELECT
+    u.id,
+    u.event_id,
+    u.user_id,
+    usr.display_name             AS uploader_name,
+    usr.is_banned,
+    usr.uploads_hidden,
+    u.preview_path,
+    u.thumbnail_path,
+    u.mime_type,
+    u.caption,
+    u.created_at,
+    COUNT(DISTINCT l.user_id)    AS like_count,
+    COUNT(DISTINCT c.id)         AS comment_count
+FROM upload u
+JOIN "user" usr ON u.user_id = usr.id
+LEFT JOIN "like" l  ON l.upload_id = u.id
+LEFT JOIN comment c ON c.upload_id = u.id AND c.deleted_at IS NULL
+WHERE u.deleted_at IS NULL
+  AND usr.uploads_hidden = FALSE
+  AND usr.is_banned = FALSE
+GROUP BY u.id, usr.display_name, usr.is_banned, usr.uploads_hidden;

backend/migrations/016_display_derivative.up.sql

@@ -0,0 +1,34 @@
+-- Display derivative: a big-screen-quality image (~2048px long edge) for the diashow.
+-- The 800px `preview_path` is sized for phone feeds (data saver); upscaled on a projector
+-- it looks soft. The diashow uses `display_path` instead — bounded in size (safe to decode
+-- on weak kiosk hardware) yet sharp on 1080p/4K. NULL until the compression worker (or the
+-- one-time backfill) generates it; consumers fall back to the original when absent.
+ALTER TABLE upload ADD COLUMN display_path TEXT;
+
+-- Recreate (not CREATE OR REPLACE, which only allows appending columns at the end) so the
+-- new column can sit alongside preview_path/thumbnail_path.
+DROP VIEW IF EXISTS v_feed;
+CREATE VIEW v_feed AS
+SELECT
+    u.id,
+    u.event_id,
+    u.user_id,
+    usr.display_name             AS uploader_name,
+    usr.is_banned,
+    usr.uploads_hidden,
+    u.preview_path,
+    u.thumbnail_path,
+    u.display_path,
+    u.mime_type,
+    u.caption,
+    u.created_at,
+    COUNT(DISTINCT l.user_id)    AS like_count,
+    COUNT(DISTINCT c.id)         AS comment_count
+FROM upload u
+JOIN "user" usr ON u.user_id = usr.id
+LEFT JOIN "like" l  ON l.upload_id = u.id
+LEFT JOIN comment c ON c.upload_id = u.id AND c.deleted_at IS NULL
+WHERE u.deleted_at IS NULL
+  AND usr.uploads_hidden = FALSE
+  AND usr.is_banned = FALSE
+GROUP BY u.id, usr.display_name, usr.is_banned, usr.uploads_hidden;