fix: production-readiness hardening from full-codebase review

Apply six isolated, low-risk fixes surfaced by the parallel
production-readiness review (backend, frontend, security, perf,
UI/UX, bugs+features).

Backend
- Mask access_token in provider GET responses and drop it on edit
  when carrying the *** placeholder — fixes plaintext leak of HA
  long-lived tokens (security H-1). Centralized via
  PROVIDER_SECRET_FIELDS so all call sites stay in sync (C-5).
- Hold HA status-change tasks in a module-level set with a
  done_callback — asyncio.create_task only keeps weak refs and
  the task could be GC'd before its row was written (C-1).
- Roll back the request session in the Telegram-webhook catch-all
  so a handler exception cannot leak uncommitted writes into the
  next request (C-2).
- Bail before reading the 1 MiB webhook body when the Gitea
  provider has no secret configured or the request has no
  signature header. For the generic webhook with bearer_token
  auth, verify the Authorization header before the body read.
  Closes the pre-auth resource-exhaustion amplifier (C-3).

Frontend
- Add supportsAutoOrganize capability to ProviderDescriptor and
  consume it from RuleEditor instead of `provider.type !== 'immich'`,
  bringing the last action-rule editor under CLAUDE.md rule 8
  (no provider-type hardcoding in components).
- Snackbar: add role="region" + per-toast role/aria-live/aria-atomic
  so screen readers announce success/error toasts.
- Sidebar nav: add aria-current="page" on the active link so the
  active state has an accessible name.
- New snackbar.region key in en + ru (locale parity preserved).

Out of scope for this commit (tracked in .claude/reviews/README.md
ship-blocker list): secret encryption at rest, JWT cookie move,
Alembic adoption, webhook idempotency, deferred-dispatch crash
window, persisted Telegram update watermark, bridge_self counter
lock — each needs more than a mechanical edit.

frontend/src/routes/+layout.svelte

@@ -499,6 +499,7 @@
 									<a
 										href={child.href}
 										class="nav-link nav-link-child group flex items-center gap-2 px-2.5 py-1.5 rounded-lg text-sm transition-all duration-200 relative {isActive(child.href) ? 'active' : ''}"
+										aria-current={isActive(child.href) ? 'page' : undefined}
 									>
 										{#if isActive(child.href)}
 											<div class="active-indicator" style="position: absolute; left: -13px; top: 50%; transform: translateY(-50%); width: 3px; height: 60%; border-radius: 0 3px 3px 0; background: var(--color-primary); box-shadow: 0 0 8px var(--color-glow-strong);"></div>
@@ -518,6 +519,7 @@
 							href={entry.href}
 							class="nav-link group flex items-center gap-2.5 {collapsed ? 'justify-center px-2' : 'px-3'} py-2 rounded-lg text-sm transition-all duration-200 relative {isActive(entry.href) ? 'active' : ''}"
 							title={collapsed ? t(entry.key) : ''}
+							aria-current={isActive(entry.href) ? 'page' : undefined}
 						>
 							{#if isActive(entry.href)}
 								<div class="active-indicator" style="position: absolute; left: 0; top: 50%; transform: translateY(-50%); width: 3px; height: 60%; border-radius: 0 3px 3px 0; background: var(--color-primary); box-shadow: 0 0 8px var(--color-glow-strong);"></div>