Add more tests

[xprilion]

This pull request adds comprehensive backend test coverage for core modules including the agent loop, app entrypoint, Celery integration, and database engine. It also improves test coverage reporting and configuration, updates documentation, and refines the test runner setup.

Testing and Coverage Improvements

• Adds pytest-cov to backend dev dependencies and configures coverage reporting in pyproject.toml for more accurate and maintainable test coverage metrics.
• Refactors the Makefile to split test-coverage into backend and frontend targets, and ensures both are run with coverage reports.
• Updates test counts in README.md to reflect the expanded backend and frontend test suites.

New Backend Test Suites

• Adds test_agent_loop.py with extensive async tests for agent tool execution, approval handling, undo/compact actions, agent turn processing, submission loop, and LLM call helpers.
• Adds test_app.py to verify FastAPI app creation, middleware, router registration, and main module entrypoint.
• Adds test_celery_app.py to validate Celery app instance, configuration, and routing.
• Adds test_db_engine.py to check database engine configuration, session creation, and context variable usage.

Test Infrastructure

• Updates backend test fixtures in conftest.py to ensure minimal app state initialization by setting app.state.config when lifespan is skipped.

[github-actions]

Qodana for Python

158 new problems were found

Inspection name	Severity	Problems
Unused imports	🔶 Warning	26
Unreachable code	🔶 Warning	1
Redundant character escape	🔶 Warning	1
Unused local symbols	◽️ Notice	57
Accessing a protected member of a class or a module	◽️ Notice	38
Unclear exception clauses	◽️ Notice	21
Shadowing names from outer scopes	◽️ Notice	8
Method is not declared static	◽️ Notice	3
PEP 8 naming convention violation	◽️ Notice	3

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