refactor(api): Pydantic request models for saved-views & feed state (R6 S1b)

- saved_views: CreateViewIn / UpdateViewIn / ReorderViewsIn. PATCH presence via
  model_fields_set; filters typed as dict (a non-object is now 422, an explicit
  null on update stays a 400 "filters must be an object").
- feed: VideoStateIn(status) / VideoProgressIn(position_seconds,duration_seconds) —
  the manual int()+400 in progress becomes a 422 on a non-number; the status
  membership 400 and the video 404 are unchanged.

Gate: ruff clean.

backend/app/routes/feed.py

@@ -5,6 +5,7 @@ import re
 from datetime import date, datetime, timedelta, timezone
 
 from fastapi import APIRouter, Depends, HTTPException, Query
+from pydantic import BaseModel
 from sqlalchemy import BigInteger, Select, and_, cast, false, func, or_, select
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.orm import Session, aliased
@@ -571,14 +572,18 @@ def get_facets(
     return {"counts": counts}
 
 
+class VideoStateIn(BaseModel):
+    status: str
+
+
 @router.post("/videos/{video_id}/state")
 def set_video_state(
     video_id: str,
-    payload: dict,
+    body: VideoStateIn,
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
-    status = payload.get("status")
+    status = body.status
     if status not in VALID_STATES:
         raise HTTPException(status_code=400, detail=f"status must be one of {VALID_STATES}")
     if db.get(Video, video_id) is None:
@@ -653,10 +658,15 @@ def clear_video_state(
     return {"video_id": video_id, "status": "new", "position_seconds": 0}
 
 
+class VideoProgressIn(BaseModel):
+    position_seconds: int = 0
+    duration_seconds: int = 0
+
+
 @router.post("/videos/{video_id}/progress")
 def set_video_progress(
     video_id: str,
-    payload: dict,
+    body: VideoProgressIn,
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
@@ -664,11 +674,8 @@ def set_video_progress(
     Stores a per-user position on the video_states row without touching watch status, so a
     partially-watched video can render a progress bar and match the "in progress" filter.
     Trivially-early and near-finished positions clear the position rather than store it."""
-    try:
-        position = int(payload.get("position_seconds") or 0)
-        duration = int(payload.get("duration_seconds") or 0)
-    except (TypeError, ValueError):
-        raise HTTPException(status_code=400, detail="position_seconds must be an integer")
+    position = body.position_seconds
+    duration = body.duration_seconds
     if position < 0:
         position = 0