feat(budgeting): budget alerts — notify when category hits 80% or 100% of monthly budget

Summary

After each sync, check if any spending category has hit 80% or 100% of its monthly budget and surface a notification via the agent.

Behaviour

• Run budget check automatically after every sync()
• Approaching (80%): "⚠️ Groceries: $340 spent of $400 budget (85%) — $60 remaining"
• Over budget (100%+): "🔴 Entertainment: $245 spent vs $200 budget — $45 over"
• Only alert once per category per month (track last-alerted timestamp)
• Suppress if no targets set

Implementation notes

• Add budget_alert_log table to track sent alerts: (user_id, line_item_id, month, threshold, alerted_at)
• Alerts surface as part of the sync summary returned to the agent
• Agent decides whether to message Ridvan based on urgency

Acceptance criteria

• Alerts included in sync response under budget_alerts key
• Deduplication: same category + month + threshold only alerts once
• Unit tests for threshold logic (79% = no alert, 80% = approaching, 100% = over)

Part of

Epic #226

Start at the sync() entry point and trace how the sync summary is assembled and where database schema changes are defined. Implement the budget_alert_log tracking and include budget_alerts in the sync response, then run or add unit tests covering 79%, 80%, and 100% thresholds plus monthly deduplication.