fix(e1): address /code-review high findings on the E1 stack

Review round on the whole E1 stack (main..HEAD). 6 findings, all low/medium,
none blocking; fixed for a clean gate-green handoff.

- plex/sync (correctness): the un-enriched cast guard compared the stored cast
  against THIS listing's length, so a trimmed Plex cast on a not-yet-enriched
  row was never applied. Compare against the listing CAP (_LISTING_CAST_CAP=20)
  instead: >cap = enriched (keep, survives a watermark reset), <=cap =
  listing-sourced (track edits in both directions). +2 regression tests.
- main (security): add X-Frame-Options: DENY as the legacy-browser fallback for
  the CSP frame-ancestors clickjacking guard.
- downloads/service (altitude): finalize_if_running now uses a SQLAlchemy 2.0
  update() construct instead of a hand-built text() UPDATE string — same atomic
  WHERE status='running', idiomatic, no identifier interpolation, no manual
  StrEnum coercion. Drops the now-unused text import.
- auth/admin (reuse): extract the SEC-10 "drop untrusted preset password + bump
  epoch" invariant into one helper (revoke_preset_password_if_unverified) so the
  Google-callback and admin-activation paths can't drift.
- tests/migrations (robustness): guarantee the shared schema is restored to head
  in a finally, so a downgrade/re-upgrade failure can't cascade into later tests.
- worker (simplification): align _finish_from_cache's display_name fill with the
  _download path's guard (no behavior change).

Gate green: ruff clean, 334 pytest (0 skip), frontend unchanged.

backend/app/main.py

@@ -164,13 +164,16 @@ async def static_cache_headers(request, call_next):
 # nosniff stops MIME-confusion, the Referrer-Policy keeps the (private) app URL from leaking to
 # external sites the user follows, `frame-ancestors 'none'` blocks click-jacking by refusing to be
 # iframed anywhere (we never embed our own pages; we embed OTHERS, which this does not affect), and
-# Permissions-Policy turns off powerful features the app never uses. A full Content-Security-Policy
+# Permissions-Policy turns off powerful features the app never uses. `X-Frame-Options: DENY` duplicates
+# the frame-ancestors intent for legacy engines that never implemented CSP frame-ancestors (belt and
+# suspenders — modern browsers use the CSP directive and ignore the header). A full Content-Security-Policy
 # (script/style/connect sources) is deferred — the SPA's inline styles + the YouTube/Plex embeds make
 # it real work — and HSTS belongs on the TLS-terminating edge (the Caddy front door), not here.
 _SECURITY_HEADERS = {
     "X-Content-Type-Options": "nosniff",
     "Referrer-Policy": "strict-origin-when-cross-origin",
     "Content-Security-Policy": "frame-ancestors 'none'",
+    "X-Frame-Options": "DENY",
     "Permissions-Policy": "camera=(), microphone=(), geolocation=()",
 }