Support dual teacher/student Telegram roles and add notification & reminder plumbing

[Despire1]

Motivation

• Allow a Telegram user to act as both Teacher and Student simultaneously without role conflicts or overwriting existing teacher data.
• Add server-side notification infrastructure to send lesson reminders, unpaid digests and student payment reminders.
• Expose manual payment reminders from the UI and ensure reminders are deduplicated/auditable.

Description

• Bot: updated src/backend/telegramBot.ts to always refresh profile data on /start via ensureTelegramUser, present role selection, persist explicit role selection via upsertTelegramUser, and permit opening the teacher mini-app when a Teacher profile exists via canOpenTeacherApp without removing student data.
• Database & migrations: extended prisma/schema.prisma with NotificationLog, added Student.isActivated/activatedAt, added Lesson.paidAt/completedAt, added User.role, and included a corresponding migration SQL (prisma/migrations/20260109094654_add_notifications/migration.sql).
• Server/notifications: added src/backend/notificationService.ts with functions to send teacher/student reminders and unpaid digests, integrated runNotificationTick() into src/backend/server.ts, and added a new API route POST /api/lessons/:lessonId/remind-payment that invokes sendStudentPaymentReminder.
• UI/client: added api.remindLessonPayment to src/shared/api/client.ts, wired remindLessonPayment through AppPage, AppRoutes, and StudentsSection, and added a remind button, spinner and handlers in StudentDebtPopoverContent and StudentHero with rate-limit/disable reasons surfaced to the user.

Testing

• No automated unit/integration tests were executed for these changes.

---

Codex Task