fix(plex): a listing cut short no longer clears watch state or Hungarian text

The daily watch reconcile reads "absent from Plex's scan" as "Plex un-watched it" and deletes the
row — but it walked the sections without asking _paginate whether the walk reached the server's
total, so a Plex hiccup mid-walk (an empty page early) un-watched every title it failed to reach
until the next complete run. The scan now reports completeness; an incomplete one still pulls in
what it saw, re-pushes dirty rows, and clears nothing (logged as plex.watch.reconcile_incomplete).

The HU overlay had the same shape: clear every row's HU text, then refill from the listing. It now
reads the listing first and clears only after a complete one. The overlay moved to plex/overlay.py
(plex/sync.py drops under the 800-line limit and off the size ratchet's list).

Tests: plex/paths (23% -> 100%: prefix boundary, longest prefix, dead mount), the reconcile's
clear/keep/re-push rules, the cut-short scan and listing (each seen red with the guard removed).

backend/tests/test_source_size.py

@@ -32,7 +32,6 @@ BIG_MODULES: dict[str, int] = {
     "app/models.py": 1607,
     "app/plex/stream.py": 1128,
     "app/plex/subtitles.py": 1092,
-    "app/plex/sync.py": 841,
     "app/routes/admin.py": 864,
     "app/routes/auth.py": 1353,
     "app/routes/downloads.py": 1237,