fix(api): throttle OAuth token requests by IP

[duyetbot]

Summary

• Reuse the existing public-endpoint limiter on OAuth token requests before parsing and credential verification.
• Default budget: five requests per minute per IP, shared with registration/project creation; honors the existing environment override.
• Returns the existing 429 RATE_LIMITED response with Retry-After and project-creation rate-limit headers. No new limiter or error schema.
• Add regression tests for failed basic/post client authentication, exhausted budgets, and IP isolation.

Closes #433.

Validation

• OAuth suite: 33 tests passed.
• API TypeScript check passed.
• Changed-file Biome check passed.

Do not merge: awaiting review per user instruction.

Co-Authored-By: Duyet Le me@duyet.net
Co-Authored-By: duyetbot bot@duyet.net

🤖 Generated with Claude Code

Summary by Sourcery

Protect the OAuth token endpoint from excessive requests while preserving the existing rate-limit behavior and response contract.

Bug Fixes:

• Throttle OAuth token requests by applying the existing per-IP public-endpoint rate limit before token processing and authentication.

Tests:

• Add regression coverage for failed basic and post client authentication, exhausted rate-limit windows, Retry-After responses, and IP isolation.

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[sourcery-ai]

Reviewer's Guide

OAuth token requests now share the existing per-IP project-creation rate limiter, enforcing the configured five-per-minute budget before parsing or credential verification while preserving the established 429 response and headers. Tests cover both client authentication methods, deterministic exhausted-window behavior, retry metadata, and isolation between IPs.

Sequence diagram for OAuth token rate limiting

sequenceDiagram
    participant Client
    participant TokenEndpoint
    participant Limiter
    participant OAuth

    Client->>TokenEndpoint: POST /token
    TokenEndpoint->>Limiter: projectCreationRateLimit
    alt IP budget exhausted
        Limiter-->>Client: 429 RATE_LIMITED with Retry-After and rate-limit headers
    else IP budget available
        Limiter-->>TokenEndpoint: Allow request
        TokenEndpoint->>OAuth: parseTokenBody
        OAuth-->>Client: Token response or authentication error
    end

Loading

File-Level Changes

Change	Details	Files
Apply the existing project-creation/public-endpoint rate limiter to OAuth token requests before request parsing and authentication.	Attach the shared limiter to POST /api/oauth/token. Use the existing per-IP five-per-minute budget and environment override. Return the existing 429 response and rate-limit headers without introducing new limiter or error types.	packages/api/src/routes/oauth/index.ts
Add regression coverage for authentication failures, exhausted rate-limit windows, retry metadata, and IP isolation.	Exercise both client_secret_basic and client_secret_post failures. Seed current and next windows to make exhaustion assertions deterministic. Verify the first request reaches authentication, the second is rate-limited, Retry-After is bounded, and another IP remains independent.	packages/api/test/oauth.test.ts

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#433	Apply per-IP rate limiting to the unauthenticated OAuth /token endpoint to reduce online brute-force attempts against client secrets.	✅
#433	Reject requests that exceed the limit with an appropriate 429 response, including retry information and existing rate-limit headers.	✅
#433	Provide regression coverage showing that failed client authentication attempts are counted, limits are enforced for both credential submission methods, and limits are isolated by IP.	✅

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