[FEATURE] Add webhook signature verification example to docs and backend tests

[ritik4ever]

Category: backend / docs
Project area: backend/src/, docs/

Context:
README documents the HMAC-SHA256 webhook signing but no automated test validates the signing logic.

Implementation:

• Unit test: verify backend signs payloads correctly with known secret.
• Integration test: deliver webhook to mock server and verify header.
• Add Node.js and Python verification examples to docs.

Acceptance Criteria:

• Signing test passes.
• Mock server rejects unsigned delivery.
• Code examples in docs are copy-paste runnable.