BE-CAL-004: implement MinimumWorkload storage

- New model: minimum_workloads table with JSON config column (per-user)
- Schemas: MinimumWorkloadConfig, MinimumWorkloadUpdate, MinimumWorkloadResponse
- Service: CRUD operations + check_workload_warnings() entry point for BE-CAL-007
- API: GET/PUT/PATCH /calendar/workload-config (self + admin routes)
- Migration: auto-create minimum_workloads table on startup
- Registered calendar router in main.py

app/api/routers/calendar.py

@@ -0,0 +1,147 @@
+"""Calendar API router.
+
+BE-CAL-004: MinimumWorkload CRUD endpoints.
+Future tasks (BE-CAL-API-*) will add slot/plan endpoints here.
+"""
+
+from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy.orm import Session
+
+from app.api.deps import get_current_user
+from app.core.config import get_db
+from app.models.models import User
+from app.schemas.calendar import (
+    MinimumWorkloadConfig,
+    MinimumWorkloadResponse,
+    MinimumWorkloadUpdate,
+)
+from app.services.minimum_workload import (
+    get_workload_config,
+    replace_workload_config,
+    upsert_workload_config,
+)
+
+router = APIRouter(prefix="/calendar", tags=["Calendar"])
+
+
+# ---------------------------------------------------------------------------
+# MinimumWorkload
+# ---------------------------------------------------------------------------
+
+@router.get(
+    "/workload-config",
+    response_model=MinimumWorkloadResponse,
+    summary="Get current user's minimum workload configuration",
+)
+def get_my_workload_config(
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Return the workload thresholds for the authenticated user.
+
+    If no configuration has been saved yet, returns default (all-zero)
+    thresholds.
+    """
+    cfg = get_workload_config(db, current_user.id)
+    return MinimumWorkloadResponse(user_id=current_user.id, config=cfg)
+
+
+@router.put(
+    "/workload-config",
+    response_model=MinimumWorkloadResponse,
+    summary="Replace the current user's minimum workload configuration",
+)
+def put_my_workload_config(
+    payload: MinimumWorkloadConfig,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Full replacement of the workload configuration."""
+    row = replace_workload_config(db, current_user.id, payload)
+    db.commit()
+    db.refresh(row)
+    return MinimumWorkloadResponse(user_id=current_user.id, config=row.config)
+
+
+@router.patch(
+    "/workload-config",
+    response_model=MinimumWorkloadResponse,
+    summary="Partially update the current user's minimum workload configuration",
+)
+def patch_my_workload_config(
+    payload: MinimumWorkloadUpdate,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Partial update — only the provided periods are overwritten."""
+    row = upsert_workload_config(db, current_user.id, payload)
+    db.commit()
+    db.refresh(row)
+    return MinimumWorkloadResponse(user_id=current_user.id, config=row.config)
+
+
+# ---------------------------------------------------------------------------
+# Admin: manage another user's workload config
+# ---------------------------------------------------------------------------
+
+def _require_admin(current_user: User = Depends(get_current_user)):
+    if not current_user.is_admin:
+        raise HTTPException(status_code=403, detail="Admin required")
+    return current_user
+
+
+@router.get(
+    "/workload-config/{user_id}",
+    response_model=MinimumWorkloadResponse,
+    summary="[Admin] Get a specific user's minimum workload configuration",
+)
+def get_user_workload_config(
+    user_id: int,
+    db: Session = Depends(get_db),
+    _admin: User = Depends(_require_admin),
+):
+    user = db.query(User).filter(User.id == user_id).first()
+    if not user:
+        raise HTTPException(status_code=404, detail="User not found")
+    cfg = get_workload_config(db, user_id)
+    return MinimumWorkloadResponse(user_id=user_id, config=cfg)
+
+
+@router.put(
+    "/workload-config/{user_id}",
+    response_model=MinimumWorkloadResponse,
+    summary="[Admin] Replace a specific user's minimum workload configuration",
+)
+def put_user_workload_config(
+    user_id: int,
+    payload: MinimumWorkloadConfig,
+    db: Session = Depends(get_db),
+    _admin: User = Depends(_require_admin),
+):
+    user = db.query(User).filter(User.id == user_id).first()
+    if not user:
+        raise HTTPException(status_code=404, detail="User not found")
+    row = replace_workload_config(db, user_id, payload)
+    db.commit()
+    db.refresh(row)
+    return MinimumWorkloadResponse(user_id=user_id, config=row.config)
+
+
+@router.patch(
+    "/workload-config/{user_id}",
+    response_model=MinimumWorkloadResponse,
+    summary="[Admin] Partially update a specific user's minimum workload configuration",
+)
+def patch_user_workload_config(
+    user_id: int,
+    payload: MinimumWorkloadUpdate,
+    db: Session = Depends(get_db),
+    _admin: User = Depends(_require_admin),
+):
+    user = db.query(User).filter(User.id == user_id).first()
+    if not user:
+        raise HTTPException(status_code=404, detail="User not found")
+    row = upsert_workload_config(db, user_id, payload)
+    db.commit()
+    db.refresh(row)
+    return MinimumWorkloadResponse(user_id=user_id, config=row.config)