fix: lazy Stripe init (unblocks test suite) + add auth/keys unit tests

- billing.ts: Stripe now initialized lazily via getStripe() instead of at module load
  This prevents test suite crash when STRIPE_SECRET_KEY env var is not set
- Add src/middleware/__tests__/auth.test.ts (6 tests): key extraction from
  Bearer header, X-API-Key header, query param; 401/403 responses; priority order
- Add src/services/__tests__/keys.test.ts (6 tests): getTierLimit for all tiers
- Total: 61 tests passing, 0 failures

src/services/__tests__/keys.test.ts

@@ -0,0 +1,28 @@
+import { describe, it, expect } from 'vitest'
+import { getTierLimit } from '../keys.js'
+
+describe('getTierLimit', () => {
+  it('should return 100 for free tier', () => {
+    expect(getTierLimit('free')).toBe(100)
+  })
+
+  it('should return 1000 for starter tier', () => {
+    expect(getTierLimit('starter')).toBe(1000)
+  })
+
+  it('should return 5000 for pro tier', () => {
+    expect(getTierLimit('pro')).toBe(5000)
+  })
+
+  it('should return 25000 for business tier', () => {
+    expect(getTierLimit('business')).toBe(25000)
+  })
+
+  it('should return 100 for unknown tier', () => {
+    expect(getTierLimit('enterprise')).toBe(100)
+  })
+
+  it('should return 100 for empty string', () => {
+    expect(getTierLimit('')).toBe(100)
+  })
+})