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/scheduler.py

@@ -2,6 +2,7 @@
 per-job run activity (from the in-process scheduler), the work still queued (DB-derived),
 and the shared quota picture."""
 from fastapi import APIRouter, Depends, HTTPException
+from pydantic import BaseModel
 from sqlalchemy import and_, func, or_, select
 from sqlalchemy.orm import Session
 
@@ -25,10 +26,14 @@ from app.sync.runner import estimate_deep_backfill
 router = APIRouter(prefix="/api/admin/scheduler", tags=["admin"])
 
 
+class JobIntervalIn(BaseModel):
+    interval_minutes: int
+
+
 @router.patch("/jobs/{job_id}")
 def set_job_interval(
     job_id: str,
-    payload: dict,
+    body: JobIntervalIn,
     admin: User = Depends(admin_user),
     db: Session = Depends(get_db),
 ) -> dict:
@@ -36,10 +41,7 @@ def set_job_interval(
     to the live scheduler immediately."""
     if job_id not in JOB_INTERVALS:
         raise HTTPException(status_code=404, detail="Unknown job")
-    try:
-        minutes = int(payload.get("interval_minutes"))
-    except (TypeError, ValueError):
-        raise HTTPException(status_code=400, detail="interval_minutes must be a number")
+    minutes = body.interval_minutes
     if not (MIN_INTERVAL <= minutes <= MAX_INTERVAL):
         raise HTTPException(
             status_code=400,
@@ -77,18 +79,19 @@ def run_all_now(user: User = Depends(admin_user)) -> dict:
     return {"started": trigger_all(actor_id=user.id)}
 
 
+class MaintenanceSettingsIn(BaseModel):
+    revalidate_batch: int
+
+
 @router.patch("/maintenance")
 def set_maintenance_settings(
-    payload: dict,
+    body: MaintenanceSettingsIn,
     admin: User = Depends(admin_user),
     db: Session = Depends(get_db),
 ) -> dict:
     """Admin-tune the maintenance job's rolling re-validation batch (videos re-checked per
     run). Persisted to app_state; the job reads it each run."""
-    try:
-        value = int(payload.get("revalidate_batch"))
-    except (TypeError, ValueError):
-        raise HTTPException(status_code=400, detail="revalidate_batch must be a number")
+    value = body.revalidate_batch
     if not (state.MAINTENANCE_BATCH_MIN <= value <= state.MAINTENANCE_BATCH_MAX):
         raise HTTPException(
             status_code=400,