fix(security): cross-event authz, SSE ticket flow, account hardening, audit logs

Follow-up to the comprehensive code review. Five batches:

1. Cross-event authorization: host_delete_upload, unban_user, and
   host_delete_comment now scope by auth.event_id. Adds
   Upload::find_by_id_and_event / soft_delete_in_event and a
   Comment::soft_delete_in_event variant that joins through upload.

2. Token exposure: SSE auth no longer puts the JWT in the URL.
   New /api/v1/stream/ticket endpoint mints a short-lived single-use
   ticket bound to the session; the EventSource passes ?ticket=...
   instead. Refuse to start in APP_ENV=production with the dev JWT
   sentinel; warn loudly otherwise.

3. Account hardening: per-IP+name rate limit on /recover (mitigates
   targeted lockout DoS), per-IP rate limit on /admin/login, random
   32-char admin recovery PIN (replaces "0000"), structured tracing
   events for wrong PIN, lockout, failed admin login, ban/unban/role
   change/pin-reset/host-delete.

4. DoS / correctness: comment listing paginated (LIMIT 50 + ?before=
   cursor), hashtag extraction whitelisted to ASCII alnum+underscore
   (≤40 chars) with unit tests, display_name / caption / comment body
   length validated in chars rather than bytes.

5. Cleanup: session-touch failures now logged, DATABASE_MAX_CONNECTIONS
   env var (default 10).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/auth/middleware.rs

@@ -43,11 +43,15 @@ impl FromRequestParts<AppState> for AuthUser {
             .map_err(|e| AppError::Internal(e.into()))?
             .ok_or_else(|| AppError::Unauthorized("Sitzung nicht gefunden oder abgelaufen.".into()))?;
 
-        // Update last_seen_at in the background (fire-and-forget)
+        // Update last_seen_at in the background (fire-and-forget). Failures are
+        // non-fatal but worth surfacing — silent swallowing hides DB connection
+        // pressure that would otherwise be the first symptom of a real problem.
         let pool = state.pool.clone();
         let session_id = session.id;
         tokio::spawn(async move {
-            let _ = Session::touch(&pool, session_id).await;
+            if let Err(e) = Session::touch(&pool, session_id).await {
+                tracing::warn!(error = ?e, session_id = %session_id, "session touch failed");
+            }
         });
 
         Ok(Self {