Preserve bot state across restarts

[ZenonEl]

Description

Active downloads, user states, and pending sends are lost on bot restart. Need to persist and restore them.

Sub-tasks

• Active downloads: Save URL + status to DB before starting, mark complete/failed after. On restart, retry pending downloads.
• User states: UserStates table already exists (PR feat(states): add UserStates table and repository for persistent states #22). Wire UserSessionManager to save/load from DB. Serialize state name + step + data JSON.
• Pending sends: If download succeeded but send failed, save to retry queue.
• Download progress: Not worth persisting — just re-download on restart.

Depends on

• feat(core): implement Result<T> pattern and error types #13 (download queue) — queue entries should be persisted
• UserStates table (already done)