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/models/minimum_workload.py

@@ -0,0 +1,66 @@
+"""MinimumWorkload model — per-user workload threshold configuration.
+
+Stores the minimum expected workload (in minutes) across four periods
+(daily / weekly / monthly / yearly) and three slot categories
+(work / on_call / entertainment).  Values are advisory: when a
+calendar submission would leave the user below these thresholds, the
+system returns a *warning* but does not block the operation.
+
+Storage decision (BE-CAL-004): independent table with a JSON column.
+This keeps the User model clean while giving each user exactly one
+configuration row.  The JSON structure matches the design document:
+
+    {
+      "daily":   {"work": 0, "on_call": 0, "entertainment": 0},
+      "weekly":  {"work": 0, "on_call": 0, "entertainment": 0},
+      "monthly": {"work": 0, "on_call": 0, "entertainment": 0},
+      "yearly":  {"work": 0, "on_call": 0, "entertainment": 0}
+    }
+
+All values are minutes in range [0, 65535].
+"""
+
+from sqlalchemy import Column, Integer, ForeignKey, JSON, DateTime
+from sqlalchemy.orm import relationship
+from sqlalchemy.sql import func
+
+from app.core.config import Base
+
+
+# Default configuration — all thresholds zeroed out (no warnings).
+DEFAULT_WORKLOAD_CONFIG: dict = {
+    "daily":   {"work": 0, "on_call": 0, "entertainment": 0},
+    "weekly":  {"work": 0, "on_call": 0, "entertainment": 0},
+    "monthly": {"work": 0, "on_call": 0, "entertainment": 0},
+    "yearly":  {"work": 0, "on_call": 0, "entertainment": 0},
+}
+
+PERIODS = ("daily", "weekly", "monthly", "yearly")
+CATEGORIES = ("work", "on_call", "entertainment")
+
+
+class MinimumWorkload(Base):
+    """Per-user minimum workload configuration."""
+
+    __tablename__ = "minimum_workloads"
+
+    id = Column(Integer, primary_key=True, index=True)
+
+    user_id = Column(
+        Integer,
+        ForeignKey("users.id"),
+        nullable=False,
+        unique=True,
+        index=True,
+        comment="One config row per user",
+    )
+
+    config = Column(
+        JSON,
+        nullable=False,
+        default=lambda: dict(DEFAULT_WORKLOAD_CONFIG),
+        comment="Workload thresholds JSON — see module docstring for schema",
+    )
+
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), onupdate=func.now())