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>

crates/picloud/tests/queue_e2e.rs

@@ -350,13 +350,14 @@ async fn queue_visibility_timeout_reclaim() {
     let (server, app_id) = server_for(pool.clone(), "vt").await;
 
     let script_id = create_script(&server, &app_id, "slow", ACK_HANDLER).await;
-    // Minimum visibility timeout per the repo's validation.
+    // Use the API's minimum visibility timeout (30s). The test below
+    // forces a stale claim that's 60s old, so it's reclaimable regardless.
     let resp = server
         .post(&format!("/api/v1/admin/apps/{app_id}/triggers/queue"))
         .json(&json!({
             "script_id": script_id,
             "queue_name": "vt_queue",
-            "visibility_timeout_secs": 5
+            "visibility_timeout_secs": 30
         }))
         .await;
     resp.assert_status(axum::http::StatusCode::CREATED);