Sync 4: push endpoint POST /api/sync/push (LWW + history snapshot)

The core conflict-resolution step. Applies a batch of client changes,
additive + owner-scoped, with last-write-wins by client edit-time — and a
version-history snapshot on every overwrite so nothing is ever lost.

client_wins(client_edited_at, server_edited_at): apply iff client >= server;
a missing client time never overwrites a real server edit; a missing server
time (new row) yields. Notes compare against updated_at; labels gain an
updated_at (migration 0017, backfilled from created_at) as their LWW field.

Notes:
- upsert with a client-supplied id: create if absent, else LWW-apply the
  full note state (title/body/color/kind/pins/trash/remind/position/items/
  manual label_ids) with the same ripple as a web edit — derive_display_title,
  _rewrite_links, _reconcile_tags (#tags), _rename_inbound_links. Overwriting
  an existing title/body snapshots the old version into note_revisions first.
  A resurrected tombstone clears purged_at.
- delete: purge tombstone (drop children + attachment files, clear content,
  set purged_at), LWW-guarded so a newer server edit survives a stale delete.

Labels: upsert (create/rename/recolor) + delete (detach from notes, tombstone),
LWW-guarded; per-owner name-uniqueness clash on a different id is rejected
rather than raising.

Response: per-item {status: created|applied|kept|noop|rejected, sync_revision};
the client pulls afterward to converge. Whole-note semantics (client sends the
full state, not a partial patch).

Tests (DB-free): client_wins across all edit-time combinations; _parse_client_dt;
push auth-guard. Apply behavior + triggers operator-verified on deploy.

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

tests/test_sync.py

@@ -1,7 +1,17 @@
+from datetime import datetime, timezone
+
 import pytest
 
 from thoughtsync.app import create_app
-from thoughtsync.sync import DEFAULT_LIMIT, MAX_LIMIT, _clamp_limit, _page_cursor, _parse_since
+from thoughtsync.sync import (
+    DEFAULT_LIMIT,
+    MAX_LIMIT,
+    _clamp_limit,
+    _page_cursor,
+    _parse_client_dt,
+    _parse_since,
+    client_wins,
+)
 
 
 @pytest.fixture
@@ -15,6 +25,31 @@ async def test_changes_requires_auth(app):
     assert resp.status_code == 401
 
 
+async def test_push_requires_auth(app):
+    client = app.test_client()
+    resp = await client.post("/api/sync/push", json={"changes": []})
+    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)
+    assert client_wins(newer, older) is True  # newer client edit wins
+    assert client_wins(older, newer) is False  # older client edit loses (server kept)
+    assert client_wins(older, older) is True  # tie → client applies (idempotent)
+    assert client_wins(None, older) is False  # unknown client time can't overwrite a real edit
+    assert client_wins(older, None) is True  # new/unknown server side yields
+    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