Implement `SignatureVerificationError`

[codebestia]

Raised by Webhook.construct_event() when the HMAC-SHA256 signature in the Shade-Signature header does not match the computed signature for the payload. Should include the expected vs received values (masked) to aid debugging.

Proposed Steps:

• Define SignatureVerificationError(ShadeError) in errors.py.
• Add header: str (received) and a hint about what went wrong (wrong secret, tampered payload).
• Do not expose the expected signature value in the exception to avoid leaking secrets.

Acceptance Criteria:

• A mismatched signature raises SignatureVerificationError.
• The exception message explains possible causes (wrong secret, payload modified in transit).
• The full expected HMAC is never included in the exception message.

[GBOYEE]

Hey, I'd like to help with this. I can handle this one. I'll fix it and add a test to lock it down. I can put up a PR for this once claimed.

[Tijesunimi004]

I have experience with Open SOurce and would like to work on this issue Maintainer

[grantfox-oss]

🦊 GrantFox — @Tijesunimi004 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #34)
2. Your PR will be reviewed by the ShadeProtocol maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@Tijesunimi004's PR #48 was approved and merged by @codebestia.

🏆 @Tijesunimi004: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (624 total points). Track your full progress on GrantFox.

👏 Great work, @Tijesunimi004! Keep contributing to ShadeProtocol.