feat(pricing): implement 4-tier pricing engine and migration (#1)

[angelTomo9]

Closes #1

Summary of Deliverables

Implements the 4-tier pricing system for x402 API calls to increase monetization efficiency by +30%.

Features & Deliverables

• Tier Structure:
  • Tier 1 (Free): Calls 1–50 ($0.00)
  • Tier 2 (Standard): Calls 51–500 ($0.01/call)
  • Tier 3 (Premium): Calls 501+ ($0.03/call)
  • Tier 4 (Priority): Flagged priority calls ($0.10/call)
• Batch Calculation: calculateBatchCost computing multi-call costs rounded to 6 decimals (USDC precision).
• SQL Migration: migrations/add_tiered_pricing.sql creating pricing_tiers table and adding tier, price_per_call, and priority_flag columns with idempotency.
• Unit Tests: tests/pricing.test.ts verifying all tier thresholds, priority overrides, and batch calculations.