fix(cache): only cache 2xx success responses in cache middleware

[singhanurag0317-bit]

🐛 Fix: Caching middleware incorrectly caches error response JSONs

Fixes #810

Problem

cacheMiddleware in backend/src/middleware/cache.middleware.js intercepted res.json and cached every response unconditionally. When a database timeout or validation failure occurred, the error response was cached and later served to subsequent valid requests.

Change

• backend/src/middleware/cache.middleware.js: the res.json override now caches only when res.statusCode is in the success range (200–299). Error responses (4xx/5xx) pass through to the client without being cached.

Tests

• New backend/src/middleware/__tests__/cacheMiddleware.test.js (4 tests):
  • caches successful 2xx responses (served from cache on the next request),
  • does not cache 4xx validation errors,
  • does not cache 5xx database-timeout errors,
  • leaves non-GET requests untouched.

Verification

• npm test in backend/ — 9 suites, 24 tests, all passing.

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