fix(stage-6): dashboard hardening + audit Lows cherry-pick

Closes 4 dashboard hardening findings and 5 of the Lows from the audit.

Dashboard hardening:
- Subtabs no longer re-fetch the app via api.apps.get on every page
  load. users/files/dead-letters drop the fetch outright (the variable
  was set but never read); queues + queues/[name] now consume the
  layout's AppContext via getContext for the page title. The layout's
  reloadApp() owns the historical-slug redirect — subtab-local redirect
  blocks are removed so there's no race.
- The global :global(details > summary::before) chevron is now scoped
  to details.chevron. The script editor's "Advanced sandbox" details
  and the inbound-email-shape help-text both opt in; the script
  exec-list logs no longer inherit a spurious chevron.
- deriveTab now matches the path segment by anchored ===, so a future
  /apps/<slug>/queues-archived route wouldn't activate the queues tab.

Lows cherry-pick:
- ExecError gains Serialize/Deserialize derives + a snake_case tag so
  RemoteExecutorClient (cluster mode v1.3+) can round-trip the variant.
- triggers_api rejects queue triggers whose visibility_timeout_secs is
  below the dispatcher's per-message executor budget; with no minimum
  the reclaim task races the handler and the queue silently
  double-delivers. Existing test using 5s updated to 30s.
- New migration 0040: execution_logs.script_id cascade switched from
  ON DELETE CASCADE to ON DELETE SET NULL so deleting a script no
  longer wipes the forensic history that motivated the delete.
- New migration 0041: dead_letters composite index on
  (app_id, created_at DESC) so the "list all" dashboard view stops
  falling back to seqscan + sort when unresolved=false.
- Schema snapshot re-blessed.

Deferred to v1.2: the ExecRequest principal serde(skip) marker
(documented in-place; the cluster-mode PR will introduce the wire-safe
snapshot at that point) and the `pic --help` mention of
`picloud admin reset-password` (one-line follow-up).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dashboard/src/routes/apps/[slug]/dead-letters/+page.svelte

@@ -1,11 +1,8 @@
 <script lang="ts">
-	import { base } from '$app/paths';
-	import { goto } from '$app/navigation';
 	import { page } from '$app/state';
-	import { api, ApiError, type App, type DeadLetterRow } from '$lib/api';
+	import { api, ApiError, type DeadLetterRow } from '$lib/api';
 
 	let slug = $derived(page.params.slug ?? '');
-	let app = $state<App | null>(null);
 	let rows = $state<DeadLetterRow[]>([]);
 	let unresolved = $state<number>(0);
 	let loading = $state(true);
@@ -13,16 +10,13 @@
 	let unresolvedOnly = $state(true);
 	let expandedId = $state<string | null>(null);
 
+	// The parent layout (apps/[slug]/+layout.svelte) already fetches the
+	// app + drives the historical-slug redirect. This page only needs the
+	// DL count and the row list — no duplicate api.apps.get().
 	async function load() {
 		loading = true;
 		error = null;
 		try {
-			const a = await api.apps.get(slug);
-			if (a.redirect_to && a.redirect_to !== slug) {
-				await goto(`${base}/apps/${a.redirect_to}/dead-letters`, { replaceState: true });
-				return;
-			}
-			app = a;
 			const c = await api.deadLetters.count(slug);
 			unresolved = c.unresolved;
 			const r = await api.deadLetters.list(slug, { unresolved: unresolvedOnly, limit: 100 });