security: replace env-or-default patterns with fail-closed helpers

[nanaf6203-bit]

Summary

The revalidate endpoint accepts a default fallback secret. Review handlers similarly use env-or-default patterns.

Affected area

• src/app/api/revalidate/route.ts
• Any other API route with fallbacks

Steps to reproduce / impact

Lead to spoofed webhooks, unauthorized cache invalidations, possible denial of cache for the entire app.

Expected behaviour

Adopt a fail-closed env-config helper that throws on missing required values in NODE_ENV === 'production'.

Acceptance criteria

• Centralised requireEnv helper used by src/app/api/**.
• CI test asserting prod build fails on missing env.

[fredericklamar342-prog]

@fredericklamar342-prog has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I can implement this Boss kindly assign

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[davidsoniaudin2-oss]

@davidsoniaudin2-oss has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

i would like to work on this issue sir

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