feat(media): authenticated signed media gateway (C1, C2-sink, C3, H2, H10)

Replaces the DB-blind `/media` ServeDir with a signed, DB-aware gateway at
`GET /media/{kind}/{id}`. Every media byte now flows through an HMAC-SHA256
signature check (minted into feed/upload DTOs for authenticated members; <img>
can't carry a Bearer header) plus a DB lookup:

- C1: export ZIP/HTML have no upload row, so they are unreachable by path —
  download stays behind the authenticated /export endpoints.
- C2 (sink): responses carry X-Content-Type-Options: nosniff and a locked-down
  CSP (default-src 'none'; sandbox), neutralizing any active content.
- C3 / H2: find_by_id filters deleted_at and the handler rejects ban-hidden
  uploaders, so deleted and moderated artifacts 404 — and the unauthenticated
  get_original alias (the H2 hole) is removed entirely.
- H10: delete paths (owner + host) now unlink original/preview/thumbnail after
  commit; soft_delete returns the paths; an hourly reaper reclaims disk for
  rows soft-deleted past a 1-day grace and hard-deletes them (FKs cascade).

Signed URLs are bucketed to a 1h window so they stay stable across feed polls
(browser cache hits) while expiring within 24h. media_token sign/verify has a
unit test (roundtrip + tamper + expiry).

Frontend: FeedUpload/pickMediaUrl now use the backend-provided signed
original_url; no client constructs a media path anymore.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/services/maintenance.rs

@@ -13,6 +13,7 @@
 //!    rate-limiter's in-memory windows (so keys for IPs that left long ago don't
 //!    accumulate).
 
+use std::path::PathBuf;
 use std::time::Duration;
 
 use sqlx::PgPool;
@@ -76,6 +77,7 @@ pub fn spawn_periodic_tasks(
     pool: PgPool,
     rate_limiter: RateLimiter,
     sse_tickets: SseTicketStore,
+    media_path: PathBuf,
 ) {
     tokio::spawn(async move {
         let mut tick = tokio::time::interval(Duration::from_secs(3600));
@@ -86,6 +88,9 @@ pub fn spawn_periodic_tasks(
             cleanup_sessions(&pool).await;
             rate_limiter.prune();
             sse_tickets.prune();
+            // Reclaim disk for uploads soft-deleted more than the grace period
+            // ago, and hard-delete those rows (FKs cascade).
+            crate::services::media_fs::reap_deleted(&pool, &media_path).await;
         }
     });
 }