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>

frontend/src/routes/host/+page.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 	import { goto } from '$app/navigation';
-	import { getToken, getRole } from '$lib/auth';
+	import { getToken, getRole, getUserId } from '$lib/auth';
 	import { api } from '$lib/api';
 	import type { MeContextDto } from '$lib/types';
 	import { onMount } from 'svelte';
@@ -57,6 +57,7 @@
 	let pinModal = $state<{ name: string; pin: string } | null>(null);
 
 	const myRole = getRole();
+	const myUserId = getUserId();
 
 	// Generic confirm-then-run for the irreversible / privilege-changing actions
 	// (promote, demote, unban, release gallery) that previously fired on one tap.
@@ -486,7 +487,11 @@
 													>
 														Entsperren
 													</button>
-												{:else}
+												{:else if user.id !== myUserId}
+													<!-- Never render target-actions (promote/demote/PIN/ban) on the
+													     caller's own row: the backend rejects every self-action
+													     (self-ban / self-demote / self-PIN) with a 400, so the button
+													     would only ever fail. -->
 													{#if user.role === 'guest' && (myRole === 'host' || myRole === 'admin')}
 														<button
 															onclick={() => (confirmAction = {