security: hardcoded fallback REVALIDATE_WEBHOOK_SECRET in API route

[nanaf6203-bit]

Summary

src/app/api/revalidate/route.ts ships with a hardcoded fallback webhook secret.

Affected area

src/app/api/revalidate/route.ts line 7

Steps to reproduce / impact

If env is missing in production, route accepts requests signed with the literal default string, rendering the HMAC useless.

Expected behaviour

Throw on missing env, fail closed. Document required env.

Acceptance criteria

• Remove literal default secret.
• Add failing test asserting rejection on absence.

[lawrettaumebe710-maker]

@lawrettaumebe710-maker has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer please can I work on this issue

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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.

hello sir i can work on this issue sir

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