feat: add maintenance_schedules table, schema, and CRUD endpoints

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/app/schemas/maintenance_schedule.py

@@ -0,0 +1,34 @@
+from datetime import datetime
+from typing import Optional
+from uuid import UUID
+from pydantic import BaseModel, Field
+
+
+class MaintenanceScheduleCreate(BaseModel):
+    tree_id: UUID
+    cron_expression: str = Field(..., min_length=9, max_length=100)
+    timezone: str = Field("UTC", max_length=100)
+    target_list_id: Optional[UUID] = None
+
+
+class MaintenanceScheduleUpdate(BaseModel):
+    cron_expression: Optional[str] = Field(None, min_length=9, max_length=100)
+    timezone: Optional[str] = Field(None, max_length=100)
+    target_list_id: Optional[UUID] = None
+    is_active: Optional[bool] = None
+
+
+class MaintenanceScheduleResponse(BaseModel):
+    id: UUID
+    tree_id: UUID
+    created_by: Optional[UUID]
+    cron_expression: str
+    timezone: str
+    target_list_id: Optional[UUID]
+    is_active: bool
+    next_run_at: Optional[datetime]
+    last_run_at: Optional[datetime]
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}