refactor(api): Pydantic request models for small route modules (R6 S1b)

payload:dict → typed request models across the single/double-endpoint modules:
- tags: TagCreateIn / TagUpdateIn (color:null clears → reads model_fields_set);
  _own_tag now delegates to owned_or_404.
- config: ConfigSetIn(value:Any) — keeps the "Missing 'value'" 400 via model_fields_set.
- me: SwitchAccountIn(user_id:int).
- scheduler: JobIntervalIn / MaintenanceSettingsIn — the manual int()+400 becomes a
  clean 422 on a non-number; the range checks stay 400.
- search: ClearSearchIn(video_ids:list[str]).
- public: WatchUnlockIn(password:str).

Gate: ruff clean, pytest 164 green.

backend/app/routes/search.py

@@ -16,6 +16,7 @@ from concurrent.futures import ThreadPoolExecutor
 from datetime import datetime, timezone
 
 from fastapi import APIRouter, Depends, HTTPException
+from pydantic import BaseModel
 from sqlalchemy import and_, delete, func, or_, select, update
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.orm import Session, aliased
@@ -323,9 +324,13 @@ def search_youtube(
     }
 
 
+class ClearSearchIn(BaseModel):
+    video_ids: list[str] = []
+
+
 @router.post("/clear")
 def clear_search(
-    payload: dict,
+    body: ClearSearchIn,
     user: User = Depends(require_human),
     db: Session = Depends(get_db),
 ) -> dict:
@@ -333,7 +338,7 @@ def clear_search(
     for them, then hard-delete the now-orphaned ones — a via_search video nobody KEPT (no watch
     state, not playlisted, channel not subscribed) and that no other user still has as a search
     find. Kept videos and organically-owned ones are left untouched."""
-    ids = [str(v) for v in (payload.get("video_ids") or [])]
+    ids = body.video_ids
     if not ids:
         return {"removed": 0}
     db.execute(