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/saved_views.py

@@ -1,4 +1,5 @@
 from fastapi import APIRouter, Depends, HTTPException
+from pydantic import BaseModel
 from sqlalchemy import func, select, update
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm import Session
@@ -40,25 +41,27 @@ def list_views(
     return [_serialize(v) for v in rows]
 
 
+class CreateViewIn(BaseModel):
+    name: str
+    filters: dict
+
+
 @router.post("")
 def create_view(
-    payload: dict,
+    body: CreateViewIn,
     user: User = Depends(require_human),
     db: Session = Depends(get_db),
 ) -> dict:
-    name = (payload.get("name") or "").strip()
+    name = body.name.strip()
     if not name:
         raise HTTPException(status_code=400, detail="name is required")
-    filters = payload.get("filters")
-    if not isinstance(filters, dict):
-        raise HTTPException(status_code=400, detail="filters must be an object")
     maxpos = db.execute(
         select(func.coalesce(func.max(SavedView.position), -1)).where(
             SavedView.user_id == user.id
         )
     ).scalar_one()
     view = SavedView(
-        user_id=user.id, name=name[:80], filters=filters, position=maxpos + 1
+        user_id=user.id, name=name[:80], filters=body.filters, position=maxpos + 1
     )
     db.add(view)
     try:
@@ -75,26 +78,34 @@ def _own_view(db: Session, user: User, view_id: int) -> SavedView:
     return owned_or_404(db, SavedView, view_id, user, detail="Unknown view")
 
 
+class UpdateViewIn(BaseModel):
+    # PATCH: presence decides what's touched (read via model_fields_set). filters, when sent,
+    # must be a real object — an explicit null is rejected (a view can't have null filters).
+    name: str | None = None
+    filters: dict | None = None
+    is_default: bool | None = None
+
+
 @router.patch("/{view_id}")
 def update_view(
     view_id: int,
-    payload: dict,
+    body: UpdateViewIn,
     user: User = Depends(require_human),
     db: Session = Depends(get_db),
 ) -> dict:
     view = _own_view(db, user, view_id)
-    if "name" in payload:
-        name = (payload.get("name") or "").strip()
+    sent = body.model_fields_set
+    if "name" in sent:
+        name = (body.name or "").strip()
         if not name:
             raise HTTPException(status_code=400, detail="name cannot be empty")
         view.name = name[:80]
-    if "filters" in payload:
-        filters = payload.get("filters")
-        if not isinstance(filters, dict):
+    if "filters" in sent:
+        if body.filters is None:
             raise HTTPException(status_code=400, detail="filters must be an object")
-        view.filters = filters
-    if "is_default" in payload:
-        make_default = bool(payload.get("is_default"))
+        view.filters = body.filters
+    if "is_default" in sent:
+        make_default = bool(body.is_default)
         if make_default:
             # At most one default per user: clear the flag on the others first.
             db.execute(
@@ -125,15 +136,17 @@ def delete_view(
     return {"deleted": view_id}
 
 
+class ReorderViewsIn(BaseModel):
+    ids: list[int]
+
+
 @router.post("/reorder")
 def reorder_views(
-    payload: dict,
+    body: ReorderViewsIn,
     user: User = Depends(require_human),
     db: Session = Depends(get_db),
 ) -> dict:
-    ids = payload.get("ids")
-    if not isinstance(ids, list):
-        raise HTTPException(status_code=400, detail="ids must be a list")
+    ids = body.ids
     rows = (
         db.execute(select(SavedView).where(SavedView.user_id == user.id))
         .scalars()