fix(audit): restore broken upload pipeline + role-based E2E audit fixes

A comprehensive role-based E2E audit (guest/host/admin, across browser
sessions) surfaced one critical and several smaller issues; this addresses
them and hardens the tests that missed them.

Critical
- The client upload pipeline was fully broken: the IndexedDB v1->v2 upgrade
  opened a *new* transaction inside the upgrade callback, which throws during
  a version-change transaction and aborted the whole upgrade, leaving the
  queue object store uncreated -- so no UI upload ever fired. Reuse the
  version-change transaction the callback provides, and bump the DB to v3 with
  a contains() guard so installs already corrupted by the shipped bug
  self-heal on next load. Re-enabled the previously-fixme'd UI upload E2E test.

High / Medium
- Event lock is uploads-only again: likes, comments and browsing stay open
  while the event is locked (USER_JOURNEYS 9.3 / FEATURES) -- it was wrongly
  freezing social interaction. Updated the event-lock spec accordingly.
- get_original now excludes soft-deleted and ban-hidden uploads, and direct
  /media/originals/** serving is blocked, so a hidden user's originals can no
  longer be pulled by UUID (all originals go through the checked alias).
- The upload handler reads the file field with an early-abort size cap chosen
  from the declared content-type, instead of buffering the entire body before
  the size check.

Low
- unban_user mirrors the ban role guard (a host can no longer unban a
  host/admin banned by an admin).
- reset_user_pin's UPDATE is event-scoped.
- Admin login returns and stores a real identity (user_id + display name)
  instead of a blank session.
- The host user list no longer renders target-actions (ban/promote/demote/PIN)
  on the caller's own row, where the backend always rejected them.
- /diashow gains a client-side auth guard like the other protected routes.
- The join page shows the event name via a new public GET /api/v1/event.

Verified: backend cargo build clean, frontend svelte-check 0 errors, full
Playwright E2E suite 144 passed / 1 skipped.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/auth/handlers.rs

@@ -238,6 +238,10 @@ pub struct AdminLoginRequest {
 #[derive(Serialize)]
 pub struct AdminLoginResponse {
     pub jwt: String,
+    /// The admin's user id + display name, so the client can populate a real identity
+    /// (own-post affordances, a name on the Account page) instead of a blank session.
+    pub user_id: Uuid,
+    pub display_name: String,
 }
 
 pub async fn admin_login(
@@ -325,7 +329,11 @@ pub async fn admin_login(
     let expires_at = Utc::now() + chrono::Duration::days(1);
     Session::create(&state.pool, admin_user.id, &token_hash, expires_at).await?;
 
-    Ok(Json(AdminLoginResponse { jwt: token }))
+    Ok(Json(AdminLoginResponse {
+        jwt: token,
+        user_id: admin_user.id,
+        display_name: admin_user.display_name,
+    }))
 }
 
 pub async fn logout(