feat(api): wallet holdings endpoint, price alert webhooks, Stellar address validation

[pre-cious-Igwealor]

Summary

• Add helper for validating Stellar wallet address format before database queries #447 — Added isValidStellarAddress(address) helper and StellarAddressSchema Zod refine schema to src/modules/wallet/wallet.utils.ts; unit tests in src/modules/wallet/__tests__/wallet.utils.test.ts
• Add wallet holdings endpoint returning all creator keys a fan owns #421 — Added GET /wallets/:address/holdings endpoint (schemas, service, controller) wired into wallets.routes.ts; integration tests mocking prisma
• Add price alert webhook registration for creator key price thresholds #423 — Added POST/GET/DELETE /alerts price alert endpoints with src/modules/alerts/ module (schemas, service, controllers, router); new prisma/schema/alert.prisma (PriceAlert model); wired into src/modules/index.ts; service unit tests

Test plan

• src/modules/wallet/__tests__/wallet.utils.test.ts — valid/invalid Stellar address cases
• src/modules/wallets/__tests__/wallet-holdings.integration.test.ts — valid address returns holdings, invalid returns 400
• src/modules/alerts/__tests__/alert.service.test.ts — create/list/delete with mocked prisma
• TypeScript: no new errors introduced (pre-existing Prisma client generation errors remain on main)

closes #421
closes #423
closes #447

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