feat: tiered pricing engine with validation, bill breakdown, and comprehensive tests

[Th3Slack3r]

Summary

Implements the Tiered Pricing Engine for issue #1 — a 4-tier pricing system for x402 API calls designed to increase revenue by ~30%.

Changes

src/pricing/tier-engine.ts — Core pricing logic:

• getTierPrice(callCount, priorityFlag) — returns price per call based on tier
• calculateBatchCost(startCount, numCalls, priority) — batch cost calculation with tier boundary crossing
• calculateBill(startCount, numCalls, priority) — full bill with per-tier breakdown
• Input validation: all functions reject NaN, Infinity, negative, fractional, and unsafe integers
• All 4 tiers: free ($0.00), standard ($0.01), premium ($0.03), priority ($0.10)

tests/pricing.test.ts — 29 unit tests covering:

• ✓ All 4 tiers with price and callsInTier verification
• ✓ Boundary crossings: free→standard, standard→premium, free→standard→premium
• ✓ Priority flag override on all tiers
• ✓ calculateBill with per-tier breakdowns
• ✓ Input validation: zero, negative, fractional, NaN, Infinity, unsafe integers
• ✓ Empty batch (zero calls) returns $0

migrations/add_tiered_pricing.sql — Idempotent SQL migration (unchanged):

• Adds tier, price_per_call, priority_flag columns to x402_calls
• Creates pricing_tiers lookup table with upsert semantics

Acceptance Criteria

• SQL migration adds tier column to relevant payment table
• Logic function get_tier_price(call_count, priority_flag) returns correct price
• Unit tests covering all 4 tiers
• No existing tests broken (29/29 pass)
• Migration is idempotent (uses IF NOT EXISTS / ON CONFLICT)

Validation

deno test tests/pricing.test.ts
ok | 29 passed | 0 failed (38ms)

Closes #1

Wallet: 0x09239d9722910bf0f6d685fb5f1af38ba660cb1d