Address PR #215 review: temporary notes hardening

Must-fix:
- CHANGELOG: add [Unreleased]/Added entry for temporary notes (#201).
- temporary-note-cleanup: re-check temporary_expires_at at deletion time so a
  concurrent "Make permanent" (sets it NULL) between the batch SELECT and the
  per-row removePage wins the race and the note is not trashed. Add unit tests
  for the make-permanent and already-trashed race windows.

Non-blocking review items:
- temporary-note-cleanup: cap the sweep batch (LIMIT 500) so a large backlog is
  not loaded into memory; remainder drains on the next hourly run.
- client: extract duplicated post-toggle cache sync into
  syncTemporaryExpiresInCache() shared by the header menu and the banner.
- Remove the tautological migration spec that mocked the whole Kysely builder.
- Tests: cover create() frozen temporaryExpiresAt (workspace override + NULL
  default fallback + non-temporary skips lookup) and restorePage disarming the
  timer (temporaryExpiresAt: null).

Deferred (forward-looking, non-blocking): extract
PageService.computeTemporaryExpiresAt() to dedupe the deadline formula and drop
the @InjectKysely from PageTemplateController; replace migration unit test with
a real Postgres up/down integration test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -32,6 +32,16 @@ per-workspace rolling-day token budget.
   foreign key is `ON DELETE SET NULL`, so deleting the target leaves a dangling
   alias any workspace member can reclaim. (#205)
 
+- **Temporary notes — auto-move to Trash after a workspace lifetime.** A note can
+  be marked temporary so it auto-moves to Trash once a configurable workspace
+  lifetime elapses (default `DEFAULT_TEMPORARY_NOTE_HOURS` = 24h) unless made
+  permanent first. The deadline is frozen at creation time, so later changes to
+  the workspace setting never reschedule existing notes; an hourly background
+  sweep trashes notes past their deadline (children ride along). An open
+  temporary note shows a banner with a "Make permanent" rescue action; restoring
+  a note from Trash disarms the timer so it is not immediately re-trashed.
+  Operators configure the lifetime per workspace. (#201)
+
 - **Persistent AI-chat history as the source of truth + server-side export.**
   An assistant turn is now persisted to the database step by step: the row is
   inserted upfront as `streaming` and updated as each agent step finishes, then