fix(review-2): high — read-only ban model, failed-compression cleanup, live SSE eviction

H1: corrected a round-1 over-reach. Bans are read-only per USER_JOURNEYS §10:
    the base AuthUser extractor no longer rejects banned users (they keep read
    access); Require{Host,Admin} and write handlers enforce the ban via
    auth.is_banned → 403 (incl. self-unban). Demoted hosts still lose powers
    immediately (live DB role) and are bounced off the dashboard. Also fixed the
    login/recover redirect regression on unauthenticated 401.

H2: failed compression now self-heals instead of leaving a permanent broken
    card — refund the quota, delete the orphaned original, soft-delete the row;
    the frontend toasts the uploader and evicts the card on upload-error.

H3: self-delete, ban-with-hide (user-hidden), and comment-deletes now broadcast
    SSE; feed, diashow, and lightbox evict the affected content live instead of
    waiting for a reload. sse-eviction.spec.ts covers it.

Riders: feed/+page.svelte also carries the medium truncated-delta pill; host.rs
also carries the low atomic release_gallery claim; admin/+page.svelte also drops
the dead compression_concurrency control (medium).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/admin/+page.svelte

@@ -57,8 +57,9 @@
 			title: 'Limits & Größen',
 			fields: [
 				{ key: 'max_image_size_mb',       label: 'Max. Bildgröße (MB)',     kind: 'number' },
-				{ key: 'max_video_size_mb',       label: 'Max. Videogröße (MB)',    kind: 'number' },
-				{ key: 'compression_concurrency', label: 'Kompressions-Worker',     kind: 'number' }
+				{ key: 'max_video_size_mb',       label: 'Max. Videogröße (MB)',    kind: 'number' }
+				// compression_concurrency is set via COMPRESSION_WORKER_CONCURRENCY at
+				// boot, not live — omitted so it isn't a dead no-op control.
 			]
 		},
 		{