Backend: Persist scheduled maintenance windows across process restarts

[Junirezz]

Context

maintenanceWindow.ts schedules future maintenance windows and auto-enables maintenance mode when a window is active (Issue #714).

Problem / Gap

Scheduled windows live in an in-memory array. Deployments, restarts, or horizontal scaling cause windows to disappear and maintenance mode transitions to drift out of sync across pods.

Proposed approach

• Persist maintenance windows in Prisma with startsAt, endsAt, title, reason, and createdBy.
• Load active/upcoming windows on startup and reconcile maintenance mode state.
• Keep the public read-only status endpoint behavior unchanged.

Acceptance criteria

• Admin-created windows survive restart and are visible from all instances.
• Active window automatically enables maintenance mode on every pod via shared storage.
• Expired windows are archived or pruned per retention policy.
• Tests cover create, overlap validation, auto-enable, and auto-disable lifecycles.

Files/areas affected

• backend/src/maintenanceWindow.ts
• backend/src/maintenanceMode.ts
• backend/prisma/schema.prisma