fix(api): a non-finite number in a JSON body is refused with a 422, not a 500

The JSON parser under FastAPI accepts `Infinity`/`NaN`, and both progress checkpoints truncate the
position with `int()` — a hand-written checkpoint came back as a 500 with a traceback. Their bodies
now refuse non-finite floats, and the app's validation handler — FastAPI's own shape — echoes a
non-finite input by name: the default one failed to serialise its own error (JSON has no NaN), which
was a 500 by another route. Sibling of the subtitle routes' non-finite query parameters.

backend/app/routes/feed.py

@@ -1,7 +1,7 @@
 from datetime import date, datetime, timezone
 
 from fastapi import APIRouter, Depends, HTTPException, Query
-from pydantic import BaseModel
+from pydantic import BaseModel, ConfigDict
 from sqlalchemy import and_, func, or_, select
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.orm import Session
@@ -303,6 +303,10 @@ def clear_video_state(
 
 
 class VideoProgressIn(BaseModel):
+    # Finite only: JSON parsing here accepts `Infinity`/`NaN`, and the handler's `int()` turned
+    # one into a 500.
+    model_config = ConfigDict(allow_inf_nan=False)
+
     # Accept a float and truncate (below) — the player may post a raw currentTime (a float), which
     # the old `int(payload.get(...))` silently truncated. A plain `int` field would 422 that on the
     # 10s progress hot path, losing the resume position; `float` keeps the old tolerance.