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

[angelTomo9]

Summary

Resolves #3 by implementing the Auto-Upsell Trigger engine triggered upon reaching 50% free call capacity.

Acceptance Criteria Checklist

• SQL Schema: upsell_triggers table defined in migrations/add_upsell_triggers.sql.
• Middleware Logic: checkAndTriggerUpsell(userId, currentCallCount, db) sets X-Upsell-Prompt: true and headers.
• A/B Prompt Variants: Dynamic selection across urgency, value, and social proof variants via getUpsellPromptVariant.
• Idempotent Execution: Triggers exactly once per user threshold crossing to prevent double-prompts.
• Unit Tests: Full test suite verifying threshold detection, header emission, and duplicate prevention in ests/upsell.test.ts.

Wallet: 0x9f8F72aA9304c8B593d555F12eF6589cC3A579A2

Closes #3