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/manager-core/tests/expected_schema.txt

@@ -181,7 +181,7 @@ table: email_trigger_details
 
 table: execution_logs
   id: uuid NOT NULL default=gen_random_uuid()
-  script_id: uuid NOT NULL
+  script_id: uuid NULL
   request_id: uuid NOT NULL
   request_path: text NULL
   request_headers: jsonb NOT NULL default='{}'::jsonb
@@ -403,6 +403,7 @@ indexes on dead_letter_trigger_details:
 
 indexes on dead_letters:
   dead_letters_pkey: public.dead_letters USING btree (id)
+  idx_dead_letters_app_created: public.dead_letters USING btree (app_id, created_at DESC)
   idx_dead_letters_app_unresolved: public.dead_letters USING btree (app_id) WHERE (resolved_at IS NULL)
   idx_dead_letters_gc: public.dead_letters USING btree (created_at)
 
@@ -589,7 +590,7 @@ constraints on email_trigger_details:
 constraints on execution_logs:
   [CHECK] execution_logs_status_check: CHECK ((status = ANY (ARRAY['success'::text, 'error'::text, 'timeout'::text, 'budget_exceeded'::text])))
   [FOREIGN KEY] execution_logs_app_id_fk: FOREIGN KEY (app_id) REFERENCES apps(id) ON DELETE CASCADE
-  [FOREIGN KEY] execution_logs_script_id_fkey: FOREIGN KEY (script_id) REFERENCES scripts(id) ON DELETE CASCADE
+  [FOREIGN KEY] execution_logs_script_id_fkey: FOREIGN KEY (script_id) REFERENCES scripts(id) ON DELETE SET NULL
   [PRIMARY KEY] execution_logs_pkey: PRIMARY KEY (id)
 
 constraints on files:
@@ -705,3 +706,5 @@ constraints on triggers:
   0037: drop idx cron triggers due
   0038: email realtime secret check
   0039: app user invitations unique pending
+  0040: execution logs keep history
+  0041: dead letters composite idx