feat: add developer portal fuzzing and rate limits

[Calebux]

This PR closes #393.
Closes #392.
Closes #385.

Summary

• Adds a developer portal at /api/developer with Swagger links, GraphQL playground, API key management, usage/rate-limit status, SDK quickstarts, and webhook testing.
• Adds per-user sensitive-operation rate limiting for borrow, withdraw, and liquidate with sliding windows, headers, graduated penalties, trusted-user bypass, and analytics.
• Extends cargo-fuzz lending coverage to liquidation and adds a scheduled/manual 30-minute fuzz workflow with crash triage and coverage artifact upload.

Verification

• npx jest src/middleware/tests/rate-limit.test.ts --runInBand --no-coverage
• ./node_modules/.bin/prettier --check api/src/app.ts api/src/middleware/rate-limit.ts api/src/routes/developer.routes.ts api/src/middleware/tests/rate-limit.test.ts
• rustfmt --check stellar-lend/fuzz/src/lending.rs
• git diff --check

Notes

• npm run typecheck --workspace api is currently blocked by pre-existing unrelated TypeScript errors in controllers, DTOs, and priceWebSocket.
• cargo fmt --all -- --check is currently blocked by unrelated workspace formatting plus a missing hello-world prop_arithmetic_test module.

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