feat: comprehensive test infrastructure improvements

- Fix unit test imports: add env setup in conftest.py before module imports
- Add 24 new auth router tests (RBAC, preferences, password validation)
- Add 16 new tests for litellm and chat_summary routers
- Apply black formatting and ruff linting across codebase
- Add pre-commit hooks configuration (black, ruff, file checks)
- Increase CI coverage threshold from 40% to 50%

Test Results:
- 206 tests passing (91 unit + 115 integration)
- Coverage: 58.79% on core modules
- auth.py: 57% → 85%, litellm.py: 23% → 87%, chat_summary.py: 41% → 100%
- auth_service: 96.51%, config: 100%, rbac: 93.48%

dashboard/backend/pyproject.toml

@@ -1,3 +1,48 @@
+[tool.black]
+line-length = 120
+target-version = ['py312']
+include = '\.pyi?$'
+extend-exclude = '''
+/(
+  # directories
+  \.eggs
+  | \.git
+  | \.hg
+  | \.mypy_cache
+  | \.tox
+  | \.venv
+  | venv
+  | _build
+  | buck-out
+  | build
+  | dist
+)/
+'''
+
+[tool.ruff]
+line-length = 120
+target-version = "py312"
+
+[tool.ruff.lint]
+select = [
+    "E",   # pycodestyle errors
+    "W",   # pycodestyle warnings
+    "F",   # pyflakes
+    "I",   # isort
+    "B",   # flake8-bugbear
+    "C4",  # flake8-comprehensions
+    "UP",  # pyupgrade
+]
+ignore = [
+    "E501",  # line too long (handled by black)
+    "B008",  # do not perform function calls in argument defaults
+    "C901",  # too complex
+    "W191",  # indentation contains tabs
+]
+
+[tool.ruff.lint.per-file-ignores]
+"__init__.py" = ["F401"]  # unused imports in __init__.py
+
 [tool.pytest.ini_options]
 minversion = "7.0"
 testpaths = ["tests"]