BE-CAL-API-006: implement plan-edit and plan-cancel API endpoints

- PATCH /calendar/plans/{plan_id}: edit a recurring schedule plan
  - Validates period-parameter hierarchy after merge
  - Rejects edits to inactive (cancelled) plans
  - Detaches future materialized slots so they keep old data
  - Past materialized slots remain untouched

- POST /calendar/plans/{plan_id}/cancel: cancel (soft-delete) a plan
  - Sets is_active=False
  - Detaches future materialized slots (plan_id -> NULL)
  - Preserves past materialized slots, returns their IDs

- Added SchedulePlanEdit and SchedulePlanCancelResponse schemas

app/schemas/calendar.py

@@ -337,3 +337,57 @@ class SchedulePlanResponse(BaseModel):
 class SchedulePlanListResponse(BaseModel):
     """Response for listing schedule plans."""
     plans: list[SchedulePlanResponse] = Field(default_factory=list)
+
+
+# ---------------------------------------------------------------------------
+# SchedulePlan edit / cancel (BE-CAL-API-006)
+# ---------------------------------------------------------------------------
+
+class SchedulePlanEdit(BaseModel):
+    """Request body for editing a recurring schedule plan.
+
+    All fields are optional — only provided fields are updated.
+    Period-parameter hierarchy (on_month → on_week → on_day) is
+    validated after merging with existing plan values.
+    """
+    slot_type: Optional[SlotTypeEnum] = Field(None, description="New slot type")
+    estimated_duration: Optional[int] = Field(None, ge=1, le=50, description="New duration in minutes (1-50)")
+    at_time: Optional[time] = Field(None, description="New daily time (HH:MM)")
+    on_day: Optional[DayOfWeekEnum] = Field(None, description="New day of week (sun-sat), use 'clear' param to remove")
+    on_week: Optional[int] = Field(None, ge=1, le=4, description="New week of month (1-4), use 'clear' param to remove")
+    on_month: Optional[MonthOfYearEnum] = Field(None, description="New month (jan-dec), use 'clear' param to remove")
+    event_type: Optional[EventTypeEnum] = Field(None, description="New event type")
+    event_data: Optional[dict[str, Any]] = Field(None, description="New event details JSON")
+    clear_on_day: bool = Field(False, description="Clear on_day (set to NULL)")
+    clear_on_week: bool = Field(False, description="Clear on_week (set to NULL)")
+    clear_on_month: bool = Field(False, description="Clear on_month (set to NULL)")
+
+    @field_validator("at_time")
+    @classmethod
+    def _validate_at_time(cls, v: Optional[time]) -> Optional[time]:
+        if v is not None and v.hour > 23:
+            raise ValueError("at_time hour must be between 00 and 23")
+        return v
+
+    @model_validator(mode="after")
+    def _at_least_one_field(self) -> "SchedulePlanEdit":
+        """Ensure at least one editable field or clear flag is provided."""
+        has_value = any(
+            getattr(self, f) is not None
+            for f in ("slot_type", "estimated_duration", "at_time", "on_day",
+                      "on_week", "on_month", "event_type", "event_data")
+        )
+        has_clear = self.clear_on_day or self.clear_on_week or self.clear_on_month
+        if not has_value and not has_clear:
+            raise ValueError("At least one field must be provided for edit")
+        return self
+
+
+class SchedulePlanCancelResponse(BaseModel):
+    """Response after cancelling a plan."""
+    plan: SchedulePlanResponse
+    message: str = Field("Plan cancelled successfully", description="Human-readable result")
+    preserved_past_slot_ids: list[int] = Field(
+        default_factory=list,
+        description="IDs of past materialized slots that were NOT affected",
+    )