fix(cache): log startup error when MemoryBackend is selected in production

[anshul23102]

Summary

Fixes #215.

`pickDefaultBackend` silently fell back to `MemoryBackend` when neither `KV_REST_API_URL+KV_REST_API_TOKEN` (Upstash) nor `REDIS_URL` (self-hosted Redis) were set. In a serverless deployment (Vercel) each Lambda invocation is an isolated process with its own in-memory store, so `MemoryBackend` provides no cross-request state sharing. Rate limits, deduplication, and any other shared-state feature were silently non-functional in production without any log evidence.

Changes

`src/lib/cache.ts`

• Adds a `console.error` in `pickDefaultBackend` that fires when `MemoryBackend` is selected and `NODE_ENV === 'production'`. The message identifies which env vars need to be configured, making the misconfiguration immediately visible in Vercel function logs.
• Exports `__pickBackendName` (test-only) — a pure function that mirrors the backend-selection logic and accepts env vars as arguments, enabling unit tests to assert backend selection without module reloading.

`src/lib/cache.test.ts`

• Adds a `backend selection` suite (6 tests) covering: Upstash selection, IoRedis selection, MemoryBackend fallback, Upstash-over-Redis priority, production warning fires, and warning absent outside production.

Behaviour

Environment	Redis config	Result
any	Upstash vars set	UpstashBackend (no warning)
any	REDIS_URL set	IoRedisBackend (no warning)
development / test	none	MemoryBackend (no warning, intended)
production	none	MemoryBackend + console.error startup log

Test plan

• npx vitest run src/lib/cache.test.ts - 27/27 pass (21 existing + 6 new)
• npx tsc --noEmit - no errors
• npx eslint src/lib/cache.ts src/lib/cache.test.ts - no errors

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[github-actions]

Hey @anshul23102

You have 5 open PRs right now. The limit is 3 at a time.

Please get your existing PRs merged or closed before opening new ones:

• fix(profile): remove OAuth token from audit/run Inngest event payload #222 - fix(profile): remove OAuth token from audit/run Inngest event payload
• fix(webhook): return generic 500 instead of leaking Supabase error fields #221 - fix(webhook): return generic 500 instead of leaking Supabase error fields
• fix: stop embedding OAuth token in Inngest audit event payload #212 - fix: stop embedding OAuth token in Inngest audit event payload
• fix: close TOCTOU race condition in claimRecommendation #211 - fix: close TOCTOU race condition in claimRecommendation

This PR will remain open but won't be reviewed until you're under the limit. See our Contributing Guidelines for details.

[anshul23102]

Temporarily closing to stay within the 3-open-PR limit. The fix is complete and tested (27/27 pass). Will reopen once one of the security PRs (#221 or #222) is merged.