ci(backend): enforce coverage gate

[emirykl]

Summary

• enforce the requested global backend thresholds: 80% lines, 70% branches, 80% functions, and 80% statements
• freeze the existing uncovered-code budget so current backend coverage cannot regress
• require files outside the baseline to satisfy the global thresholds before merge
• run coverage against the unit suite plus the verification lifecycle E2E suite with Redis and an ephemeral PostgreSQL database
• fix the pre-existing invalid testnet workflow secret condition that blocked Actions from parsing workflows on the branch

Validation

• 49 test suites passed
• 456 tests passed
• backend lint and build passed
• an intentionally uncovered module failed the global gate, confirming new modules cannot bypass coverage
• GitHub Backend CI passed on the final commit

Closes #277

[kilodesodiq-arch]

Hi 👋 Appreciate this PR! The CI is flagging a failing check — could you take a peek at the failed job and push a fix? Happy to help if you need a hand 🙏

[emirykl]

Thanks for flagging this. I rebased onto the latest main and updated the coverage job to use an ephemeral PostgreSQL service, since the backend schema has moved from SQLite to PostgreSQL. I also refreshed the coverage baseline for the current source tree. The final commit now passes Backend CI (49 suites, 456 tests) and all PR checks are green.