[Backend] backend/src/middleware/rate-limiter.middleware.ts and stream-rate-limiter.middleware.ts duplicate the same window/store setup logic

[grantfox-oss]

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Why this matters

backend/src/middleware/rate-limiter.middleware.ts and backend/src/middleware/stream-rate-limiter.middleware.ts both configure an express-rate-limit instance with a store and window, presumably repeating the same Redis/memory-store selection logic and key-generation boilerplate. Duplicated configuration means a fix to one (e.g. the trust-proxy IP-keying bug already tracked) has to be manually mirrored in the other.

Acceptance criteria

• Extract a shared createRateLimiter(options) factory used by both middlewares
• Migrate both files to use the shared factory
• Confirm existing rate-limiter tests still pass unchanged

Files to touch

• backend/src/middleware/rate-limiter.middleware.ts
• backend/src/middleware/stream-rate-limiter.middleware.ts

Out of scope

• Fixing the trust-proxy IP-keying issue itself (tracked separately)
• Switching rate-limit backends