fix: close the nine ways an unattended event loses photos or dies

Every one of these was found in the pre-event audit, verified against source, and
survives to production on the current main. Grouped by what actually goes wrong.

PHOTOS DISAPPEAR

* compression.rs no longer soft-deletes on a failed derivative. The guest got a
  201, watched the card appear, then watched it vanish — the row left v_feed,
  find_visible_media and BOTH keepsakes, while its bytes sat on disk for 14 days
  waiting for a cleanup nothing announced. No screen anywhere lists compression
  failures, so recovery meant hand-written SQL that also had to re-add the
  refunded quota. Now it does exactly what the ENOSPC arm beside it already did
  and documented as correct: keep the row, serve the original, retry on the next
  boot (bounded by derivative_attempts). `upload-deleted` is no longer emitted;
  `upload-processed` is, so the card re-renders instead of sitting on a
  placeholder.

* A 413 is now a reversible lock, so the blob survives. The quota moves — free
  disk falls, uploader count rises — so a guest goes over it having done nothing,
  and treating that as permanent meant a 400 MB video was pushed across cellular
  in full and THEN deleted from IndexedDB. Gone on both sides, and unrecoverable
  for an in-app camera capture that exists nowhere else.

* quota_limit_bytes gained a floor and a stable divisor. The ceiling used to
  decrease monotonically all evening; it now settles at max(uploaders,
  estimated_guest_count) — a config key that was seeded, validated in the admin
  whitelist, and read by no code at all. The floor is clamped to what the disk
  can actually back, so a full volume still yields zero rather than handing out
  an allowance it cannot honour.

* Because that floor gives up the aggregate guarantee the formula used to imply,
  uploads now check a hard 10 GB reserve first, independent of every quota
  toggle. postgres_data, media_data and exports_data share one filesystem: the
  end state was not a degraded feature, it was Postgres unable to write WAL.

THE ARCHIVE DISAPPEARS

* prune_superseded_archives runs only after the new generation lands. It ran
  before the preflight, reasoning the old archive was already unreachable — true
  of reachability, false of recoverability. An epoch is a value that can be
  rolled back; deleted bytes cannot. Any failed rebuild left the event with NO
  keepsake at all.

* The export preflight reserves the same 10 GB. `free < needed` authorised an
  export sized at exactly free, which ran for half an hour and landed the box at
  zero with the keepsake still unfinished.

THE APP DIES

* The feed reconcile re-reads the id set after its awaits instead of reusing one
  captured up to three round-trips earlier. The new-upload SSE handler prepends
  during exactly that window, so the row was both already present and absent from
  the stale set — prepended twice, and a duplicate key in a keyed {#each} throws
  in production, not just dev. The SSE handler and loadMore now dedupe too.

* Added routes/+error.svelte. Without it any uncaught error fell through to
  SvelteKit's unstyled English 500 with no reload control — inside a chromeless
  standalone PWA with no URL bar, for the rest of the evening.

THE OPERATOR IS LOCKED OUT

* admin_login verifies the password BEFORE charging the rate bucket, and a
  correct password is never throttled. The old order made this a denial of
  service against its own operator: every guest shares one NAT IP, the check ran
  first, so five requests a minute from any phone in the room kept the bucket
  full — and the escape hatch needed the admin session being blocked. A generous
  separate ceiling still bounds bcrypt CPU.

THE PROJECTOR DIES

* The preload budget is now strictly inside the dwell. At the 3s option the 4s
  budget could never land a commit on a slow uplink, so the wall froze on one
  photo while the queue drained silently behind it.

* The wake lock retries every 30s while visible, and the page says so on screen
  when the browser has no wake lock API. visibilitychange was the only retry
  trigger and a kiosk never changes visibility, so one refusal — iOS in Low Power
  Mode, say — was permanent.

* Caddy: /api/v1/upload/*/display joins the cacheable carve-out. The backend set
  max-age=300 on it and the blanket no-store silently replaced it, so a projector
  re-fetched a full-size JPEG per slide, ~2-4 GB over an evening on the uplink
  the guests are uploading over.

Also removes Upload::soft_delete, now unreferenced and an unscoped footgun next
to soft_delete_in_event.

Verified: 146/146 backend tests against a live Postgres, clippy clean, 51/51
vitest, svelte-check 0 errors, eslint clean, vite build, caddy validate, compose
YAML parse.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Caddyfile

@@ -31,20 +31,27 @@
 	@hashed_assets path_regexp hashed /_app/immutable/.*\.[a-f0-9]{8,}\.(js|css|woff2)$
 	header @hashed_assets Cache-Control "public, max-age=31536000, immutable"
 
-	# Preview/thumbnail images. These are served by the app through a visibility-checked
-	# alias (/api/v1/upload/{id}/{preview,thumbnail}) so moderation can revoke access;
-	# the app serves no /media route at all, so there is no direct path to the bytes.
-	# Privately cacheable for a short window (the app sets the same header; this is the
-	# edge carve-out from the blanket no-store below). Kept short so a moderated image
-	# stops being served to a direct-URL holder promptly.
-	@media_api path /api/v1/upload/*/preview /api/v1/upload/*/thumbnail
+	# Preview/thumbnail/display images. These are served by the app through a
+	# visibility-checked alias (/api/v1/upload/{id}/{preview,thumbnail,display}) so
+	# moderation can revoke access; the app serves no /media route at all, so there is no
+	# direct path to the bytes. Privately cacheable for a short window (the app sets the
+	# same header; this is the edge carve-out from the blanket no-store below). Kept short
+	# so a moderated image stops being served to a direct-URL holder promptly.
+	#
+	# `display` was missing here while the backend set `private, max-age=300` on it, and
+	# because `header` REPLACES, the blanket no-store below silently won. That route is the
+	# ~2048px derivative the diashow uses exclusively, so a projector left running all
+	# evening re-fetched a full-size JPEG for every slide — roughly 2-4 GB pulled through
+	# the app over 8 hours, on the same venue uplink 100 guests are uploading over, and a
+	# blank frame on every network hiccup.
+	@media_api path /api/v1/upload/*/preview /api/v1/upload/*/thumbnail /api/v1/upload/*/display
 	header @media_api Cache-Control "private, max-age=300"
 
 	# API and health — never cache, EXCEPT the gated image routes above. A cached health
 	# response would report the last known state rather than the current one.
 	@api {
 		path /api/* /health
-		not path /api/v1/upload/*/preview /api/v1/upload/*/thumbnail
+		not path /api/v1/upload/*/preview /api/v1/upload/*/thumbnail /api/v1/upload/*/display
 	}
 	header @api Cache-Control "no-store"