feat: Phase 1 Group 6 — add account_id to maintenance_schedules

Primary backfill: tree_id → trees.account_id
Fallback: created_by → users.account_id (for is_default tree rows)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/maintenance_schedule.py

@@ -23,6 +23,12 @@ class MaintenanceSchedule(Base):
     created_by: Mapped[Optional[uuid.UUID]] = mapped_column(
         UUID(as_uuid=True), ForeignKey("users.id", ondelete="SET NULL"), nullable=True
     )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
     cron_expression: Mapped[str] = mapped_column(String(100), nullable=False)
     timezone: Mapped[str] = mapped_column(String(100), nullable=False, default="UTC")
     target_list_id: Mapped[Optional[uuid.UUID]] = mapped_column(