feat(config): type-safe config module with Zod validation

[Uchechukwu-Ekezie]

Closes #11

What changed

Zod schema (src/config/config.schema.ts)

• Full schema covering every env var with coercion (z.coerce), URL validation, string minimums, enum checks, and defaults
• CORS_ALLOWED_ORIGINS transforms a comma-separated string into string[]
• STELLAR_NETWORK_PASSPHRASE is optional (derived from the network enum at runtime)
• validate() export: crashes with a descriptive, field-level error on invalid input; in production all defaultable keys must be explicitly set (no silent fallbacks); in development missing defaults emit a console.warn

Config module (src/config/config.module.ts)

• Wraps @nestjs/config's ConfigModule.forRoot with isGlobal: true, cache: true, and the Zod validate hook
• AppModule imports it first so validation runs at bootstrap — misconfiguration surfaces immediately, not mid-request

All process.env.* reads migrated to ConfigService

• CheckoutService, StellarService, RedisService, AuthService, PaymentDetectorService, WebhookQueueService, DynamicCorsMiddleware — all now inject ConfigService<Config> with { infer: true } for full TypeScript type inference

config:check script

• scripts/config-check.ts — calls validate(process.env) and exits 0/1
• "config:check": "ts-node scripts/config-check.ts" added to package.json — useful for CI/CD and Docker health checks

.env.example

• Fully rewritten; every variable marked REQUIRED or OPTIONAL with a one-line description and example value

Tests (src/config/config.schema.spec.ts — 14 tests)

• Valid config passes; defaults applied; PORT coerced from string; CORS split and trimmed
• Missing DATABASE_URL, short JWT_SECRET, invalid URL, non-G receiving account, unknown STELLAR_NETWORK — all rejected
• validate() throws a descriptive field-level message on invalid input
• Production rejects defaultable keys; production passes when all keys explicitly provided

How to test

npm test               # 264 tests, all passing
npm run config:check   # validate env vars without starting the app

[oomokaro1]

LGTM — type-safe config module with Zod validation. Merge conflicts resolved, only minor lint issues remaining (extra blank lines in checkout.service.spec.ts). Approving to unblock merge.