M9 S4: sync adopts serialization/parse_dt toolkit + normalizes push oracle

DRY:
- serialize.py: serialize_label_sync(label) = base serialize_label + the
  delta-only fields (sync_revision/purged_at/created_at via iso()). sync's
  changes() adopts it; the local _serialize_label_row near-dup is gone.
- sync adopts common.parse_dt (drops the byte-identical _parse_client_dt;
  4 call sites) and common.iso for the note delta augmentation. (Manual-label
  reconciliation was already shared in S3.) Fully folding the note
  re-augmentation into the serializer waits on the notes.py split.
- test_sync: drops the now-redundant _parse_client_dt test (parse_dt is
  covered in test_notes) + its dead import.

Security (issue — push existence-oracle): a foreign-owned id on push was
rejected with "not yours", distinguishing "another user's note" from a free
id. A legit client only pushes ids of notes it created, so that branch is
only hit by a probe (or ~0-prob UUID collision) — now a GENERIC "cannot apply"
rejection that doesn't confirm the id exists. The residual create-vs-reject
status difference is inherent to client-chosen ids over a global PK and is
practically unexploitable (a shared note already exposes its id to recipients).

Sync behavior operator-verified on deploy (no Postgres CI lane).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

tests/test_sync.py

@@ -8,7 +8,6 @@ from thoughtsync.sync import (
     MAX_LIMIT,
     _clamp_limit,
     _page_cursor,
-    _parse_client_dt,
     _parse_since,
     client_wins,
 )
@@ -42,14 +41,6 @@ def test_client_wins():
     assert client_wins(None, None) is True
 
 
-def test_parse_client_dt():
-    assert _parse_client_dt("2026-07-22T00:00:00Z").year == 2026
-    assert _parse_client_dt("2026-07-22T00:00:00+00:00").tzinfo is not None
-    assert _parse_client_dt("garbage") is None
-    assert _parse_client_dt(None) is None
-    assert _parse_client_dt(123) is None
-
-
 def test_parse_since():
     assert _parse_since(None) == 0
     assert _parse_since("42") == 42