feat(calendar): maintenance window + schedule_type special slots

## What this adds

1. **Maintenance window on ScheduleType**
   - New columns: maintenance_from / maintenance_to (UTC hours, 0-23)
   - Invariant: window is exactly 1 hour (validated in pydantic;
     maintenance_to must equal (maintenance_from + 1) % 24)
   - Default applied via additive migration: 8:00-9:00 UTC for existing
     rows so deployments don't crash on first boot

2. **ScheduleTypeSpecialSlot** — admin-managed slot template
   - New table schedule_type_special_slots
   - Admin (schedule_type.manage) CRUD via
     /schedule-types/{id}/special-slots
   - Fields: name, description, minute_in_window (0-59 inside the
     parent maintenance window), estimated_duration, priority,
     event_data (JSON merged into materialised slot), is_active
   - Unique constraint (schedule_type_id, name) — name is the stable
     human-readable identifier per cohort

3. **Per-agent materialisation**
   - New service app/services/special_slot_materialiser.py
   - GET /calendar/sync calls materialise_special_slots_for_claw
     (idempotent, one row per agent per template per date)
   - GET /calendar/day calls materialise_special_slots_for_user
   - Materialised rows are slot_type=system, event_type=system_event,
     is_admin_locked=true, special_slot_id pointing back to template
   - Plugin's runSync picks them up like any other due slot via the
     normal real-slots query path

4. **Admin-locked enforcement**
   - New TimeSlot columns: is_admin_locked, special_slot_id (FK to
     schedule_type_special_slots, ON DELETE SET NULL)
   - PATCH /calendar/slots/{id}: refuses any edit on admin-locked
     slots (423)
   - POST /calendar/slots/{id}/cancel: refuses cancel on admin-locked
     (423)
   - PATCH /calendar/slots/{id}/agent-update: admin-locked accept only
     ongoing/paused/finished/aborted statuses (423 on other transitions)

5. **Maintenance-window guard on slot creation**
   - POST /calendar/slots: rejects slot_type=system outright (only
     materialiser may create system slots) and rejects any non-system
     slot whose [scheduled_at, +duration] intersects the calling
     user's schedule_type maintenance window (422). Handles 23->0 wrap

6. **Schema response**
   - TimeSlotResponse / CalendarSlotItem now include is_admin_locked
     and special_slot_id so clients can render the lock indicator and
     trace back to the template

## Migration

Additive only — no destructive changes. Lives in _migrate_schema()
in app/main.py; the new schedule_type_special_slots table is created
by Base.metadata.create_all() on first boot.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/models/calendar.py

@@ -178,11 +178,37 @@ class TimeSlot(Base):
         comment="Source plan if materialized from a SchedulePlan; set NULL on edit/cancel",
     )
 
+    # -----------------------------------------------------------------
+    # Admin-locked slots are materialised from a ScheduleTypeSpecialSlot
+    # template. The agent can complete / abort / pause / resume them but
+    # cannot edit their time, type, duration, or cancel them outright —
+    # the slot exists because admin decided every agent on the parent
+    # schedule_type should run it. See `_apply_agent_slot_update` for
+    # the enforcement.
+    # -----------------------------------------------------------------
+    is_admin_locked = Column(
+        Boolean,
+        nullable=False,
+        server_default="0",
+        comment="True for slots materialised from a schedule_type special slot template.",
+    )
+
+    # Pointer back to the template that materialised this slot. NULL for
+    # all user-created or plan-generated slots. Lets us cascade updates
+    # and surface 'why is this on my calendar' to the agent.
+    special_slot_id = Column(
+        Integer,
+        ForeignKey("schedule_type_special_slots.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+
     created_at = Column(DateTime(timezone=True), server_default=func.now())
     updated_at = Column(DateTime(timezone=True), onupdate=func.now())
 
     # relationship ----------------------------------------------------------
     plan = relationship("SchedulePlan", back_populates="materialized_slots")
+    special_slot = relationship("ScheduleTypeSpecialSlot")
 
 
 # ---------------------------------------------------------------------------