fix(security): harden authz, media gating, recover, SSE, deps

Security audit follow-up. No Critical/High existed; these close the
Medium/Low findings.

F1 [Med] Host privilege boundary: a host could demote a peer host to
  guest and then ban / PIN-reset (→ account takeover via /recover) them,
  because the ban/pin-reset peer guards key off the target's *current*
  role. set_role now blocks a non-admin from demoting a host.

F2 [Med] Moderation now revokes preview/thumbnail access: they are served
  through visibility-checked aliases (/api/v1/upload/{id}/{preview,
  thumbnail}) that filter soft-deleted / ban-hidden uploads, and direct
  /media/previews|thumbnails is 404-blocked. Feed emits the gated URLs;
  Caddy keeps them privately cacheable (max-age=300, short so moderation
  revokes promptly — the live feed already evicts cards via SSE). Uses a
  lean find_visible_media() (paths+mime only) on the media hot path.

F3 [Med/Low] npm audit fix: svelte 5.55.1→5.56.4 (SSR-XSS advisories),
  devalue 5.6.4→5.8.1 (DoS). Prod deps: 0 vulnerabilities.

F4 [Low] /recover no longer leaks account existence: unknown display name
  returns the same 401 as a wrong PIN, and runs a dummy bcrypt verify so
  timing matches — closing the enumeration + timing oracle.

F5 [Low] SSE streams re-validate the session every ~60s and close once it
  is logged out / expired, instead of running until client disconnect.

F6 [Info] X-Content-Type-Options: nosniff set at the app layer on all
  media responses (defense-in-depth alongside the edge).

Tests: new e2e specs for F1 (host cannot demote peer host), F2 (preview
gated + 404 after delete + direct /media blocked), F4 (uniform 401).
40 backend unit tests + 182 e2e passing.

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

backend/src/handlers/host.rs

@@ -219,9 +219,14 @@ pub async fn set_role(
     .await?
     .ok_or_else(|| AppError::NotFound("Benutzer nicht gefunden.".into()))?;
 
-    if target.0 == "admin" {
+    // Admins are untouchable by hosts. A plain host also may not demote another
+    // *host*: without this guard a host could demote a peer host to guest and then
+    // ban / PIN-reset (→ account-takeover via /recover) them — the ban/pin-reset peer
+    // guards key off the target's *current* role, so a prior demotion would launder
+    // past them. Only an admin may change a host's role.
+    if target.0 == "admin" || (target.0 == "host" && auth.role != UserRole::Admin) {
         return Err(AppError::Forbidden(
-            "Admins können nicht geändert werden.".into(),
+            "Du darfst die Rolle dieses Benutzers nicht ändern.".into(),
         ));
     }