ci: add SDK test workflow and path-filter backend CI

[MasRama]

Adds a separate GitHub Actions workflow for the SDK test suite and tightens the existing backend workflow so it only triggers on backend or workflow changes. Also adds CI status badges to the README.

PR #28 already migrated the backend tests to pytest and set up the initial workflow. This PR keeps that work and scopes it to finverify-terminal/backend/** plus the workflow file itself.

Closes #7

Local verification:

• pytest -q in finverify-terminal/backend: 189 passed
• python -m tests.test_dvl in finverify-terminal/sdk: 11 passed
• Both workflow YAML files validated with yaml.safe_load

[vercel]

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[aadityat23]

Thanks for another great contribution! Really appreciate you taking the time to review the previous PR, identify the remaining scope, and improve the CI configuration instead of duplicating work. The path-filtering optimization and separate SDK workflow are nice additions. Merged! Looking forward to more contributions!

[MasRama]

Thanks for the merge, @aadityat23! Happy to help.