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/schemas/calendar.py

@@ -0,0 +1,63 @@
+"""Calendar-related Pydantic schemas.
+
+BE-CAL-004: MinimumWorkload read/write schemas.
+"""
+
+from __future__ import annotations
+
+from pydantic import BaseModel, Field, model_validator
+from typing import Optional
+
+
+# ---------------------------------------------------------------------------
+# MinimumWorkload
+# ---------------------------------------------------------------------------
+
+class WorkloadCategoryThresholds(BaseModel):
+    """Minutes thresholds per slot category within a single period."""
+    work: int = Field(0, ge=0, le=65535, description="Minutes of work-type slots")
+    on_call: int = Field(0, ge=0, le=65535, description="Minutes of on-call-type slots")
+    entertainment: int = Field(0, ge=0, le=65535, description="Minutes of entertainment-type slots")
+
+
+class MinimumWorkloadConfig(BaseModel):
+    """Full workload configuration across all four periods."""
+    daily: WorkloadCategoryThresholds = Field(default_factory=WorkloadCategoryThresholds)
+    weekly: WorkloadCategoryThresholds = Field(default_factory=WorkloadCategoryThresholds)
+    monthly: WorkloadCategoryThresholds = Field(default_factory=WorkloadCategoryThresholds)
+    yearly: WorkloadCategoryThresholds = Field(default_factory=WorkloadCategoryThresholds)
+
+
+class MinimumWorkloadUpdate(BaseModel):
+    """Partial update — only provided periods/categories are overwritten.
+
+    Accepts the same shape as ``MinimumWorkloadConfig`` but every field
+    is optional so callers can PATCH individual periods.
+    """
+    daily: Optional[WorkloadCategoryThresholds] = None
+    weekly: Optional[WorkloadCategoryThresholds] = None
+    monthly: Optional[WorkloadCategoryThresholds] = None
+    yearly: Optional[WorkloadCategoryThresholds] = None
+
+
+class MinimumWorkloadResponse(BaseModel):
+    """API response for workload configuration."""
+    user_id: int
+    config: MinimumWorkloadConfig
+
+    class Config:
+        from_attributes = True
+
+
+# ---------------------------------------------------------------------------
+# Workload warning (used by future calendar validation endpoints)
+# ---------------------------------------------------------------------------
+
+class WorkloadWarningItem(BaseModel):
+    """A single workload warning returned alongside a calendar mutation."""
+    period: str = Field(..., description="daily | weekly | monthly | yearly")
+    category: str = Field(..., description="work | on_call | entertainment")
+    current_minutes: int = Field(..., ge=0, description="Current scheduled minutes in the period")
+    minimum_minutes: int = Field(..., ge=0, description="Configured minimum threshold")
+    shortfall_minutes: int = Field(..., ge=0, description="How many minutes below threshold")
+    message: str = Field(..., description="Human-readable warning")