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/services/minimum_workload.py

@@ -0,0 +1,144 @@
+"""MinimumWorkload service — CRUD and validation helpers.
+
+BE-CAL-004: user-level workload config read/write + future validation entry point.
+"""
+
+from __future__ import annotations
+
+import copy
+from typing import Optional
+
+from sqlalchemy.orm import Session
+
+from app.models.minimum_workload import (
+    DEFAULT_WORKLOAD_CONFIG,
+    CATEGORIES,
+    PERIODS,
+    MinimumWorkload,
+)
+from app.schemas.calendar import (
+    MinimumWorkloadConfig,
+    MinimumWorkloadUpdate,
+    WorkloadWarningItem,
+)
+
+
+# ---------------------------------------------------------------------------
+# Read
+# ---------------------------------------------------------------------------
+
+def get_workload_config(db: Session, user_id: int) -> dict:
+    """Return the raw config dict for *user_id*, falling back to defaults."""
+    row = db.query(MinimumWorkload).filter(MinimumWorkload.user_id == user_id).first()
+    if row is None:
+        return copy.deepcopy(DEFAULT_WORKLOAD_CONFIG)
+    return row.config
+
+
+def get_workload_row(db: Session, user_id: int) -> Optional[MinimumWorkload]:
+    """Return the ORM row or None."""
+    return db.query(MinimumWorkload).filter(MinimumWorkload.user_id == user_id).first()
+
+
+# ---------------------------------------------------------------------------
+# Write (upsert)
+# ---------------------------------------------------------------------------
+
+def upsert_workload_config(
+    db: Session,
+    user_id: int,
+    update: MinimumWorkloadUpdate,
+) -> MinimumWorkload:
+    """Create or update the workload config for *user_id*.
+
+    Only the periods present in *update* are overwritten; the rest keep
+    their current (or default) values.
+    """
+    row = db.query(MinimumWorkload).filter(MinimumWorkload.user_id == user_id).first()
+
+    if row is None:
+        row = MinimumWorkload(
+            user_id=user_id,
+            config=copy.deepcopy(DEFAULT_WORKLOAD_CONFIG),
+        )
+        db.add(row)
+
+    # Merge provided periods into existing config
+    current = copy.deepcopy(row.config) if row.config else copy.deepcopy(DEFAULT_WORKLOAD_CONFIG)
+
+    for period in PERIODS:
+        period_data = getattr(update, period, None)
+        if period_data is not None:
+            current[period] = period_data.model_dump()
+
+    # Ensure JSON column is flagged as dirty for SQLAlchemy
+    row.config = current
+    db.flush()
+    return row
+
+
+def replace_workload_config(
+    db: Session,
+    user_id: int,
+    config: MinimumWorkloadConfig,
+) -> MinimumWorkload:
+    """Full replace of the workload config for *user_id*."""
+    row = db.query(MinimumWorkload).filter(MinimumWorkload.user_id == user_id).first()
+
+    if row is None:
+        row = MinimumWorkload(user_id=user_id, config=config.model_dump())
+        db.add(row)
+    else:
+        row.config = config.model_dump()
+
+    db.flush()
+    return row
+
+
+# ---------------------------------------------------------------------------
+# Validation entry point (BE-CAL-007 will flesh this out)
+# ---------------------------------------------------------------------------
+
+def check_workload_warnings(
+    db: Session,
+    user_id: int,
+    scheduled_minutes: dict[str, dict[str, int]],
+) -> list[WorkloadWarningItem]:
+    """Compare *scheduled_minutes* against the user's configured thresholds.
+
+    ``scheduled_minutes`` has the same shape as the config:
+        {"daily": {"work": N, ...}, "weekly": {...}, ...}
+
+    Returns a list of warnings for every (period, category) where the
+    scheduled total is below the minimum.  An empty list means no warnings.
+
+    This is the entry point that BE-CAL-007 and the calendar API endpoints
+    will call.
+    """
+    config = get_workload_config(db, user_id)
+    warnings: list[WorkloadWarningItem] = []
+
+    for period in PERIODS:
+        cfg_period = config.get(period, {})
+        sch_period = scheduled_minutes.get(period, {})
+        for cat in CATEGORIES:
+            minimum = cfg_period.get(cat, 0)
+            if minimum <= 0:
+                continue
+            current = sch_period.get(cat, 0)
+            if current < minimum:
+                shortfall = minimum - current
+                warnings.append(WorkloadWarningItem(
+                    period=period,
+                    category=cat,
+                    current_minutes=current,
+                    minimum_minutes=minimum,
+                    shortfall_minutes=shortfall,
+                    message=(
+                        f"{period.capitalize()} {cat.replace('_', '-')} workload "
+                        f"is {current} min, below minimum of {minimum} min "
+                        f"(shortfall: {shortfall} min)"
+                    ),
+                ))
+
+    return warnings