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/handlers/upload.rs

@@ -99,9 +99,11 @@ pub async fn upload(
     let mime = content_type.unwrap_or_else(|| "application/octet-stream".to_string());
     let size = data.len() as i64;
 
-    // Validate caption length
+    // Validate caption length. Counted in chars (code points) to match the
+    // "Zeichen" wording in the error message — `.len()` would be bytes and
+    // reject perfectly valid German/emoji captions early.
     if let Some(ref cap) = caption {
-        if cap.len() > MAX_CAPTION_LENGTH {
+        if cap.chars().count() > MAX_CAPTION_LENGTH {
             return Err(AppError::BadRequest(format!(
                 "Beschreibung ist zu lang. Maximum: {} Zeichen.",
                 MAX_CAPTION_LENGTH