Verify SendGrid webhook signatures

[hman38705]

Area: Backend
Files: services/api/src/email/webhook.rs, services/api/src/security.rs, services/api/src/config.rs

Problem: Webhook events are accepted without signature validation.

Acceptance Criteria:

• Signature/timestamp verification is enforced.
• Invalid signatures are rejected with 401 or 403.
• Replay protection window is implemented.

[daree-dev]

@daree-dev has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

want to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @daree-dev to this issue.

[k2ghostyou]

@k2ghostyou has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

i would like to work on this isssue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @k2ghostyou to this issue.

[drips-wave]

Congratulations, @k2ghostyou! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

🧑‍💻 @k2ghostyou: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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