feat: persist and serve attestations via repository and cache layer

[Vivian-04]

PR #321 [Backend] Wire attestations endpoint to repository and remove empty-array placeholder in app.ts

Summary

Wires the public attestation API to the real repository/cache/outbox path instead of the placeholder handlers in src/app.ts.

Changes

• Mounts src/routes/attestations.ts from src/app.ts.
• Adds repository-backed paginated list support with accurate totals.
• Uses read-through attestation cache for list reads and invalidates cache after writes.
• Persists new attestations in a transaction and emits an attestation.created outbox event.
• Handles duplicate attestations with 409.
• Normalizes Ethereum addresses before repository/cache use.
• Validates payload size for key and value.
• Documents the endpoints in docs/api.md and docs/openapi.yaml.
• Extends route/cache/schema/envelope coverage for create, pagination, duplicates, normalization, and cache invalidation.

Verification

Passing focused verification:

npx.cmd vitest run tests/routes/attestations.test.ts src/services/__tests__/attestationCacheService.test.ts src/schemas/attestations.test.ts src/__tests__/envelopeContract.test.ts

Result: 4 passed, 46 tests passed.

Repo-wide verification still has unrelated existing failures:

• npm.cmd run test fails in env config, member mocks, API key/webhook audit logs, bulk verification, rate limit, and Horizon listener tests.
• npm.cmd run build fails on unrelated TypeScript errors across Soroban retry types, config timeout fields, outbox metrics, webhook repository interface, audit/service types, key manager types, and other existing modules.

The attestation-related webhook event typing issue was fixed by adding attestation.created and attestation.revoked to WebhookEventType.

Closes #321

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