[FEATURE] Add backend unit test coverage to 80%+

[ritik4ever]

Category: testing / backend
Project area: backend/src/tests/

Implementation:

• Audit current coverage.
• Add tests for: stream lifecycle, indexer, webhook delivery, auth middleware.
• Target >= 80% branch coverage.

Acceptance Criteria:

• Coverage >= 80% branches.
• CI fails if coverage drops.
• Coverage badge in README.