[Feature] Track-level deadline subscriptions via existing push/subscription model

[jerry609]

Context

Follow-up slice for Epic #154.

The next smallest useful step is not a separate conference reminder system. It should extend the existing subscription / push stack with track-level deadline reminders derived from the current deadline radar matching.

Scope

• attach Subscribe this track's deadlines to the existing track workflow
• support 3 reminder channels: Dashboard, Email, Telegram
• support 3 lead-time windows: 30 / 14 / 7 days
• support 3 frequencies: daily summary / approaching only / changes only
• keep automatic deadline matching on by default; subscription is an explicit reminder layer

Reuse

• src/paperbot/infrastructure/services/subscription_service.py
• src/paperbot/infrastructure/push/telegram_subscription_store.py
• src/paperbot/api/routes/push_commands.py
• src/paperbot/api/routes/research.py deadline radar matching

Acceptance Criteria

• a track can persist deadline reminder preferences without introducing a separate subscription system
• dashboard can surface subscribed reminders from the existing push/subscription model
• email and telegram reminders are opt-in per track
• reminders use current track-to-deadline matching and respect the selected windows/frequency

Notes

• this issue intentionally stays below epic scope
• Obsidian integration work in [Epic] Obsidian CLI 集成 — 研究知识管理 #159 should be split separately; it is not bundled into this slice