test: add backend coverage reporting

[saurabhhhcodes]

What changed

• Added coverage to the shared dependency list so contributors can run coverage locally.
• Added backend/.coveragerc to exclude virtual environments and migrations from coverage reporting.
• Documented the coverage run manage.py test and coverage html workflow in CONTRIBUTING.md.

Why

• Issue LO-054 [Easy]: Add Code Coverage Reports to Backend Test Suite #391 asks for a beginner-friendly way to measure backend test coverage and generate reports.
• The setup makes it easier to spot missing tests without changing application behavior.

Validation

• git diff --check

Closes #391

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