feat: implement token bucket rate limiting with burst handling and ti…

[distributed-nerd]

feat: Token bucket rate limiting with burst handling and tier awareness

Replaces the fixed-window tieredRateLimit with a proper token bucket algorithm that supports burst handling, per-endpoint limits, user tier differentiation, and distributed state via Redis.

Changes

backend/src/middleware/rate-limit.ts (new)

• Token bucket implementation with configurable capacity, refillRate, and burstAllowance per tier
• Tokens refill continuously at refillRate/s up to capacity + burstAllowance
• Redis path uses an atomic Lua script — safe for distributed/multi-instance deployments
• Falls back to in-memory store on Redis failure (fail-open) with a console warning to handle clock skew and Redis outages gracefully
• Sets X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset, X-RateLimit-Policy, X-RateLimit-Tier headers on every response
• Returns 429 with Retry-After header and structured error body when bucket is empty
• In-memory ring-buffer analytics (recordAnalyticsEvent, getAnalyticsSummary)

backend/src/routes/rate-limit-analytics.ts (new)

• GET /api/v1/rate-limit/analytics?windowMs=60000
• Returns total/blocked request counts broken down by tier and endpoint

backend/src/index.ts

• Removed old fixed-window tieredRateLimit and express-rate-limit invoice limiter
• Wired tokenBucketRateLimit as the global /api/ middleware
• Invoice route gets its own stricter bucket config via endpointConfig override
• Registered analytics route under /api/v1/rate-limit

backend/.env.example

• Added commented-out vars for REDIS_URL and per-tier burst/refill config

Default tier limits

Tier	Capacity	Refill	Burst
free	60	1 tok/s	10
pro	300	5 tok/s	50
enterprise	1200	20 tok/s	200

Per-endpoint overrides

Endpoint	Free capacity	Pro capacity	Enterprise capacity
/api/v1/invoice	10	60	300
/api/v1/verification	20	100	500
/api/v1/stellar	30	150	600

Edge cases handled

• Redis failure — fails open, logs warning, continues with in-memory
• Clock skew — refill uses elapsed ms delta, not wall-clock windows
• Burst traffic — one-time burst allowance above capacity, then normal refill applies
• Distributed instances — atomic Lua script ensures no race conditions on Redis

closes #224

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[Smartdevs17]

⚠️ Merge conflicts detected

Hi @distributed-nerd,

This PR has merge conflicts that must be resolved before it can be merged.

To fix:

1. Sync with the latest main branch: git fetch origin && git merge origin/main
2. Resolve the conflicts in your code
3. Push the resolved changes

Once resolved, the PR will be automatically merged on the next run.

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🤖 Drips Wave Review Agent

[Smartdevs17]

⚠️ Merge conflicts detected

Hi @distributed-nerd,

This PR has merge conflicts that must be resolved before it can be merged.

To fix:

1. Sync with the latest main branch: git fetch origin && git merge origin/main
2. Resolve the conflicts in your code
3. Push the resolved changes

Once resolved, the PR will be automatically merged on the next run.

---

🤖 Drips Wave Review Agent

[Smartdevs17]

⚠️ Merge conflicts detected

Hi @distributed-nerd,

This PR has merge conflicts that must be resolved before it can be merged.

To fix:

1. Sync with the latest main branch: git fetch origin && git merge origin/main
2. Resolve the conflicts in your code
3. Push the resolved changes

Once resolved, the PR will be automatically merged on the next run.

---

🤖 Drips Wave Review Agent

[gitguardian]

⚠️ GitGuardian has uncovered 1 secret following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

Since your pull request originates from a forked repository, GitGuardian is not able to associate the secrets uncovered with secret incidents on your GitGuardian dashboard.
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🔎 Detected hardcoded secret in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
32145778	Triggered	Generic Password	aaed709	backend/src/relayer/tests/signature.test.ts	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secret safely. Learn here the best practices.
3. Revoke and rotate this secret.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

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^{🦉 GitGuardian detects secrets in your source code to help developers and security teams secure the modern development process. You are seeing this because you or someone else with access to this repository has authorized GitGuardian to scan your pull request.}

[distributed-nerd]

@Smartdevs17 conflicts resolved