docs: add backend coverage reporting

[saurabhhhcodes]

What changed

• Added coverage to the project requirements.
• Added a backend .coveragerc that excludes migrations and virtual environments.
• Documented the local coverage workflow in CONTRIBUTING.md.

Why

• Contributors need a simple, repeatable way to measure which backend areas are covered by tests.
• This makes it easier to spot missing coverage before opening a PR.

Validation

• git diff --check

Closes #391

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