feat(monetization): implement auto-upsell trigger engine (#3)

[jihadMo]

Closes #3

Summary of Changes:

• Migration: Created migrations/add_upsell_triggers.sql with idempotent upsell_triggers table tracking user_id, trigger_type, shown_at, converted, and prompt_variant.
• Engine Implementation: Implemented src/monetization/upsell.ts with UpsellEngine class evaluating call volume and returning X-Upsell-Prompt: true with contextual prompt copy on reaching 50% free threshold (5th call out of 10).
• Idempotency: Guaranteed trigger fires strictly once per threshold crossing.
• Tests: Added unit test suite in tests/upsell.test.ts covering threshold detection (calls 1-4 vs call 5) and duplicate suppression.

Wallet for Base USDC settlement:

0x3AD489c09A5C750b29155B0d0426Fc70654D59e5