feat: per-task USD budget cap for background tasks

[arniesaha]

Problem

Background tasks (scraping, deploys, analysis) are spawned with no spend limit. A runaway loop or unexpectedly deep task could burn significant API budget before anyone notices.

Solution

Add maxBudgetUsd param to background task dispatch:

• Default cap: e.g. $0.50 per task
• When cost exceeds cap: cancel task, log reason, notify via Telegram
• Task journal records budget_cap and actual_cost per task

Acceptance Criteria

• Background tasks accept optional maxBudgetUsd param
• Task is cancelled when cost exceeds cap
• Telegram notification sent on budget exceeded
• Task journal records budget cap + actual spend