fix(api): accept fractional progress seconds (R6 S1b code-review round 1)

/code-review high round 1 on S1b — 5 findings, 1 fixed, 4 by-design/deferred:
- FIXED: VideoProgressIn / ItemProgressIn position_seconds & duration_seconds
  typed `int` would 422 a raw float currentTime on the 10s progress checkpoint
  (the old `int(payload.get(...))` truncated it). Back to `float`, truncated in
  the handler; +test. The live frontend floors so this was latent, not live.
By-design / deferred (no change): the 422 detail-array isn't localized by the FE
(filed under R7); PATCH is-not-none (channels/quota, null-safe) vs model_fields_set
(nullable-clear) is a deliberate split; missing-field→422 is the point of the epic;
me/switch_account's numeric-string coercion is more correct and still session-gated.

Gate: ruff clean, pytest 172 green.

backend/app/routes/plex.py

@@ -1943,8 +1943,10 @@ def _push_watch(
 
 
 class ItemProgressIn(BaseModel):
-    position_seconds: int = 0
-    duration_seconds: int = 0
+    # float, truncated in the handler — the player may post a raw currentTime (a float); a plain
+    # `int` field would 422 it on the periodic progress checkpoint (the old `int(...)` truncated).
+    position_seconds: float = 0
+    duration_seconds: float = 0
     final: bool = False
 
 
@@ -1962,8 +1964,8 @@ def item_progress(
     debounces the Plex resume push: we mirror the settled position to Plex only on a final flush, so
     a single watch doesn't spray `/:/timeline` calls at the server every 10 seconds."""
     it = _item_or_404(db, rating_key)
-    position = max(0, body.position_seconds)
-    duration = body.duration_seconds
+    position = max(0, int(body.position_seconds))
+    duration = int(body.duration_seconds)
     final = body.final
     now = datetime.now(timezone.utc)
     st = db.query(PlexState).filter_by(user_id=user.id, item_id=it.id).first()