feat(monetization): implement auto-upsell trigger engine and A/B prompt variants (#3)

[angelTomo9]

Closes #3

Summary of Deliverables

Implements the auto-upsell trigger engine in src/monetization/upsell.ts with SQL migration and comprehensive test suite.

Features & Deliverables

• Database Schema: migrations/add_upsell_triggers.sql creating upsell_triggers table with indexing on user_id and trigger_type.
• Auto-Upsell Engine: src/monetization/upsell.ts providing:
  • 50% threshold detection (5th free call out of 10 limit)
  • X-Upsell-Prompt: true header signaling
  • A/B tested dynamic prompt variants (urgency vs value)
  • Strict idempotency (fires once per user threshold crossing)
  • Conversion tracking (markConversion).
• Test Suite: tests/upsell.test.ts verifying pre-threshold suppression, 5th-call activation, double-trigger prevention, and conversion updates.