feat: add per-org and per-IP rate limit tiers

[smartalee]

Summary

Adds per-org rate limiting alongside existing per-IP limits for org-scoped routes.

Changes

• Added ORG_RATE_LIMIT_MAX and ORG_RATE_LIMIT_WINDOW_MS to env config
• Modified orgAuth.ts to attach orgId to request object
• Updated rate limiter key generator to include org ID in rate limit keys
• Applied org-aware rate limiters to /org/:orgId/vaults and /org/:orgId/analytics routes
• Added matrix tests for per-org/per-IP rate limiting
• Added documentation in README

Testing

npm test -- rateLimiter.tiers.test.ts

Closes #341

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