Expire trash after 30 days, and make the deadline something you can see

Trash had no end. A note sat in /trash until someone emptied it by hand, and
its attachment BYTES sat on disk the whole time — the pile-up the operator
asked about. Nothing purged; there was no scheduler at all.

Retention is server-owned: `trash_retention_days` (default 30, 0 = keep
forever) in the settings registry, so it lands in admin Settings with no
migration and takes effect without a restart. A background sweep started in
before_serving does the work. Clients learn about a purge the way they learn
about any deletion — as a tombstone on the delta feed.

An auto-purge nobody can see coming is data loss on a timer, so the window is
now visible: /api/config publishes it, notes carry `deleted_at`, Trash leads
with the policy, and each card counts down. The countdown rounds DOWN — saying
"1 day left" for a note with ten minutes on the clock is the one error here
that actually costs someone a note.

Three things this turned up on the way:

- `DELETE /api/notes/<id>` hard-deleted the row, leaving no tombstone at all.
  A permanent delete in the web UI never reached a linked device, which would
  keep its copy forever and push it back on the next edit. It now purges
  through the same path as everything else.
- The purge left `note_revisions` and `note_link_previews` behind. A revision
  holds the full body, so the text of a "permanently deleted" note was still
  sitting in the database.
- `deleted_at` now SURVIVES a purge instead of being cleared. It's still true,
  and it means every query that says "not trashed" excludes tombstones for
  free — without it a content-less row reads as a perfectly normal active note
  and shows up on the board as a blank card.

Desktop keeps its own clock only when there's nobody else to keep one: the
sweep runs at startup on an UNLINKED device and refuses otherwise. A linked
client that expired notes on its own schedule could destroy something the
server was deliberately keeping, then push that delete upstream. Local policy
must never outrank the server's — so it also adopts the server's window for
the countdown rather than showing its offline default.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SreJkbxB4gx8pPsu8QbLPi

src/thoughtsync/settings.py

@@ -43,6 +43,15 @@ REGISTRY: list[SettingDef] = [
         "How long a signed-in session stays valid before another login is required.",
         "Access",
     ),
+    SettingDef(
+        "trash_retention_days",
+        "int",
+        30,
+        "Trash retention (days)",
+        "How long a note stays in Trash before it's permanently deleted, freeing its "
+        "attachments from disk. Set to 0 to keep trashed notes until they're deleted by hand.",
+        "Notes",
+    ),
     SettingDef(
         "max_attachment_mb",
         "int",
@@ -117,11 +126,16 @@ async def get_setting(db, key: str) -> Any:
 
 
 async def get_public_config(db) -> dict:
-    """Non-sensitive settings the unauthenticated login/register screen needs."""
+    """Non-sensitive settings every client reads — the login/register screen before
+    sign-in, and the app itself afterwards. Nothing here is owner-scoped."""
     return {
         "site_name": await get_setting(db, "site_name"),
         "allow_registration": await get_setting(db, "allow_registration"),
         "enable_url_unfurl": await get_setting(db, "enable_url_unfurl"),
+        # Server policy, not user data: clients need it to say how long a note has
+        # left in Trash. A native client also reads it BEFORE linking, which is why
+        # it belongs on the unauthenticated config rather than behind login.
+        "trash_retention_days": await get_setting(db, "trash_retention_days"),
     }