attachments sync: attach offline, upload when linked, removals stick (#5168)

Desktop could not create an attachment at all, and a removed attachment or
dismissed preview came back on the next pull. Now:

- core: add_attachment keeps the bytes in the blob store and queues the row
  (schema v10: attachments.uploaded / upload_error). Push uploads it once its
  note has landed. A refusal that retrying won't fix (too large, id clash, hash
  mismatch) is recorded on the file and not re-sent every cycle; the editor
  shows it.
- core: removing a synced attachment or dismissing a preview leaves a tombstone
  in pending_deletes; push sends it as an `attachment`/`preview` delete, and a
  pull while it waits doesn't put the row back. A pull also keeps files still
  waiting to upload instead of replacing them wholesale.
- server: PUT /api/sync/attachments/<id> (raw body, sha256-checked, idempotent,
  size-capped) and child deletes in push, which apply regardless of LWW and
  answer noop for rows the caller can't see. One store_attachment helper for
  the upload route, the importer and sync. Protocol 5, feature attachment_sync;
  the client sends neither to a server without it.
- server: migration 0031 makes a link preview's insert/delete bump its note, so
  background-fetched previews and web dismissals reach linked devices.
- desktop: Attach and paste-image work offline (raw-bytes IPC command).
- SVG is served as a download by the desktop blob scheme too (as #1981 did for
  the web), and drawn as a file chip on both.
- autosync: drop the catch_unwind; release builds abort on panic, so it only
  ever worked in debug builds.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_sync.py

@@ -34,6 +34,12 @@ async def test_push_requires_auth(app):
     assert resp.status_code == 401
 
 
+async def test_attachment_upload_requires_auth(app):
+    client = app.test_client()
+    resp = await client.put("/api/sync/attachments/00000000-0000-0000-0000-000000000000", data=b"x")
+    assert resp.status_code == 401
+
+
 def test_client_wins():
     older = datetime(2026, 7, 20, tzinfo=timezone.utc)
     newer = datetime(2026, 7, 22, tzinfo=timezone.utc)