fix(upload): stop destroying originals, apply EXIF orientation, surface rejections

Three defects in the same pipeline, each of which loses a photo or misrepresents
one.

1. A transient error destroyed the guest's only copy.

`process`'s error arm unconditionally `remove_file`d the original. Every failure
routed there: `create_dir_all`, both derivative `save_with_format` calls (disk
full is the canonical case, and it arrives exactly when many guests upload at
once), a panic inside the image codec, or a momentary DB-pool exhaustion. The
row is only SOFT-deleted, so the bytes were the sole unrecoverable part — and
they were the part we deleted. The author already knew this was wrong next door:
`backfill_missing_display` says it "must NEVER soft-delete an upload that already
has a working preview".

Retry up to 3 times with backoff (re-checking the e2e generation guard after each
sleep), and on final failure keep the refund + soft-delete but leave the original
on disk, logging its path. A failed upload is now recoverable instead of gone.

2. Every portrait photo was stored sideways.

Phones don't rotate sensor data — they record the camera orientation in EXIF and
store the pixels as shot. `decode()` returns those raw pixels and the JPEG
re-encode writes no EXIF, so the 800px preview, the 2048px diashow display and
the keepsake were all rotated 90°, while "Original anzeigen" rendered upright
because the original keeps its tag. That asymmetry is why it reads as a viewer
bug. There was no EXIF handling anywhere in the repo and no exif crate.

Read the tag via `into_decoder()` (which carries the decode Limits through, so
the decompression-bomb cap is untouched) and apply it. Missing/malformed tags
fall back to NoTransforms — most images have none.

Existing derivatives are already baked wrong, so migration 018 adds
`derivatives_rev` and `backfill_missing_display` becomes
`backfill_stale_derivatives`: it now also picks up anything below the current rev
and regenerates it once from the original, which still carries its EXIF. Videos
are marked current in the migration — ffmpeg already honours the rotation matrix.
Bump DERIVATIVES_REV for any future change that invalidates derivatives.

3. A rejected upload vanished without a word.

`UploadQueue.svelte` — 162 lines holding the ONLY renderer of an item's error
text, the only "Erneut" retry button and the only rate-limit countdown — was
never imported anywhere, so `retryItem`, `removeItem` and `clearCompleted` were
unreachable at runtime. On a terminal rejection the store purged the blob and
wrote a clear German reason into `entry.error` "so the UI shows a clear reason".
There was no such UI. And `uploadBadgeCount` counted only pending/uploading, so
the badge decremented exactly as if the upload had succeeded.

Mount the queue on /upload, toast the reason immediately (the flow sends the user
to /feed straight after staging, so the list alone would still miss them), and
count blocked/error in the badge so a failure can't read as success.

Tests: 02-upload/exif-orientation uploads a 40x20 fixture tagged Orientation=6
and asserts both derivatives come back PORTRAIT, with a sanity check that the
source really is stored landscape. 02-upload/rejection-visible bans the uploader
between staging and sending, then asserts the toast, the queue row with the
server's reason, and that the item is still counted.

Note: 02-upload/quota's 4 failures are pre-existing and unrelated — see the next
commit.

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

frontend/src/lib/upload-queue.ts

@@ -3,6 +3,7 @@ import { writable, get } from 'svelte/store';
 import { getToken, getUserId, clearAuth } from './auth';
 import { onSseEvent } from './sse';
 import { refreshQuota } from './quota-store';
+import { toast } from './toast-store';
 
 export interface QueueItem {
 	id: string;
@@ -629,6 +630,11 @@ async function uploadItem(id: string): Promise<void> {
 			entry.error = e.message;
 			await database.put(STORE_NAME, entry);
 			updateItemStatus(id, 'blocked', e.message);
+			// Tell the user NOW. The queue list only lives on /upload, and the flow sends
+			// them straight to /feed after staging a photo — so without this a rejected
+			// upload was silently swallowed: the blob is gone, the FAB badge drops exactly
+			// as if it had succeeded, and the photo simply never appears.
+			toast(`${entry.fileName}: ${e.message}`, 'error');
 			return;
 		}
 		const msg = e instanceof Error ? e.message : 'Upload fehlgeschlagen.';