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/migrations/0040_execution_logs_keep_history.sql

@@ -0,0 +1,18 @@
+-- Audit Low finding: preserving forensic history.
+--
+-- The `execution_logs.script_id` foreign key was created with ON DELETE
+-- CASCADE in 0001_init. Deleting a script then wipes every log row that
+-- ever referenced it — including the rows that captured the failure
+-- that motivated the delete. Switch to ON DELETE SET NULL so the
+-- forensic history survives and operators can still look up "what did
+-- this dead script do before we removed it" by id.
+
+ALTER TABLE execution_logs
+    DROP CONSTRAINT IF EXISTS execution_logs_script_id_fkey;
+
+ALTER TABLE execution_logs
+    ALTER COLUMN script_id DROP NOT NULL;
+
+ALTER TABLE execution_logs
+    ADD CONSTRAINT execution_logs_script_id_fkey
+    FOREIGN KEY (script_id) REFERENCES scripts(id) ON DELETE SET NULL;

crates/manager-core/migrations/0041_dead_letters_composite_idx.sql

@@ -0,0 +1,12 @@
+-- Audit Low finding: speeding up the "list all DL for app" view.
+--
+-- The dashboard's dead-letters tab issues
+--   SELECT ... FROM dead_letters
+--     WHERE app_id = $1
+--     ORDER BY created_at DESC LIMIT $2
+-- which falls back to a seq-scan + sort when the unresolved=false
+-- filter is on (the partial idx_dead_letters_app_unresolved doesn't
+-- cover resolved rows). This composite covers both modes.
+
+CREATE INDEX IF NOT EXISTS idx_dead_letters_app_created
+    ON dead_letters (app_id, created_at DESC);