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/models/comment.rs

@@ -40,15 +40,35 @@ impl Comment {
         .await
     }
 
-    pub async fn list_for_upload(pool: &PgPool, upload_id: Uuid) -> Result<Vec<CommentDto>, sqlx::Error> {
+    /// Paginated comment listing — returns up to `limit` rows in chronological
+    /// order (oldest first). If `before` is set, only comments older than that
+    /// timestamp are returned, enabling backward cursor pagination ("load
+    /// earlier"). Without the LIMIT a hot post with thousands of comments could
+    /// OOM the server on a single GET.
+    pub async fn list_for_upload(
+        pool: &PgPool,
+        upload_id: Uuid,
+        before: Option<DateTime<Utc>>,
+        limit: i64,
+    ) -> Result<Vec<CommentDto>, sqlx::Error> {
+        // Two-step: pick the newest `limit` rows older than `before`, then flip
+        // them back into ascending order so the caller can render top-to-bottom.
         sqlx::query_as::<_, CommentDto>(
-            "SELECT c.id, c.upload_id, c.user_id, u.display_name AS uploader_name, c.body, c.created_at
-             FROM comment c
-             JOIN \"user\" u ON u.id = c.user_id
-             WHERE c.upload_id = $1 AND c.deleted_at IS NULL
-             ORDER BY c.created_at ASC",
+            "SELECT * FROM (
+               SELECT c.id, c.upload_id, c.user_id, u.display_name AS uploader_name,
+                      c.body, c.created_at
+               FROM comment c
+               JOIN \"user\" u ON u.id = c.user_id
+               WHERE c.upload_id = $1 AND c.deleted_at IS NULL
+                 AND ($2::timestamptz IS NULL OR c.created_at < $2)
+               ORDER BY c.created_at DESC
+               LIMIT $3
+             ) page
+             ORDER BY created_at ASC",
         )
         .bind(upload_id)
+        .bind(before)
+        .bind(limit)
         .fetch_all(pool)
         .await
     }
@@ -69,4 +89,25 @@ impl Comment {
             .await?;
         Ok(())
     }
+
+    /// Event-scoped variant of [`Self::soft_delete`]. Returns `false` if the
+    /// comment doesn't exist or belongs to a different event.
+    pub async fn soft_delete_in_event(
+        pool: &PgPool,
+        id: Uuid,
+        event_id: Uuid,
+    ) -> Result<bool, sqlx::Error> {
+        let result = sqlx::query(
+            "UPDATE comment
+             SET deleted_at = NOW()
+             WHERE id = $1
+               AND deleted_at IS NULL
+               AND upload_id IN (SELECT id FROM upload WHERE event_id = $2)",
+        )
+        .bind(id)
+        .bind(event_id)
+        .execute(pool)
+        .await?;
+        Ok(result.rows_affected() > 0)
+    }
 }