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/social.rs

@@ -1,6 +1,7 @@
-use axum::extract::{Path, State};
+use axum::extract::{Path, Query, State};
 use axum::http::StatusCode;
 use axum::Json;
+use chrono::{DateTime, Utc};
 use serde::Deserialize;
 use uuid::Uuid;
 
@@ -51,12 +52,24 @@ pub async fn toggle_like(
     Ok(StatusCode::NO_CONTENT)
 }
 
+#[derive(Deserialize, Default)]
+pub struct ListCommentsQuery {
+    /// RFC3339 timestamp — return only comments older than this. Pass the
+    /// `created_at` of the oldest currently-loaded comment to fetch the next
+    /// older page.
+    pub before: Option<DateTime<Utc>>,
+}
+
+const COMMENT_PAGE_SIZE: i64 = 50;
+
 pub async fn list_comments(
     State(state): State<AppState>,
     _auth: AuthUser,
     Path(upload_id): Path<Uuid>,
+    Query(q): Query<ListCommentsQuery>,
 ) -> Result<Json<Vec<CommentDto>>, AppError> {
-    let comments = Comment::list_for_upload(&state.pool, upload_id).await?;
+    let comments =
+        Comment::list_for_upload(&state.pool, upload_id, q.before, COMMENT_PAGE_SIZE).await?;
     Ok(Json(comments))
 }
 
@@ -79,7 +92,8 @@ pub async fn add_comment(
     }
 
     let text = body.body.trim();
-    if text.is_empty() || text.len() > 500 {
+    let text_chars = text.chars().count();
+    if text_chars == 0 || text_chars > 500 {
         return Err(AppError::BadRequest(
             "Kommentar muss zwischen 1 und 500 Zeichen lang sein.".into(),
         ));