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/state.rs

@@ -4,6 +4,7 @@ use tokio::sync::broadcast;
 use crate::config::AppConfig;
 use crate::services::compression::CompressionWorker;
 use crate::services::rate_limiter::RateLimiter;
+use crate::services::sse_tickets::SseTicketStore;
 
 #[derive(Clone, Debug)]
 pub struct SseEvent {
@@ -29,6 +30,7 @@ pub struct AppState {
     pub sse_tx: broadcast::Sender<SseEvent>,
     pub compression: CompressionWorker,
     pub rate_limiter: RateLimiter,
+    pub sse_tickets: SseTicketStore,
 }
 
 impl AppState {
@@ -42,6 +44,7 @@ impl AppState {
             sse_tx,
             compression,
             rate_limiter: RateLimiter::new(),
+            sse_tickets: SseTicketStore::new(),
         }
     }
 }