test: add comprehensive test suite with 89% coverage

- Add tests for all backend modules: config, database, models, services,
  routes, middleware, and providers
- Separate dev dependencies using dependency-groups
- Update CLAUDE.md with test commands and project structure
- Add .coverage and .pytest_cache to gitignore

88 tests covering:
- Authentication and authorization
- Model routing and key selection
- Async logging with batch processing
- All API endpoints (chat, openai, anthropic, health)
- LiteLLM wrapper (mocked external calls)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/tests/test_auth.py

@@ -0,0 +1,47 @@
+"""Tests for authentication service."""
+import pytest
+from unittest.mock import patch, AsyncMock
+from app.services.auth import AuthService
+from app.models import ClientKey
+
+
+class TestAuthService:
+    """Test AuthService."""
+
+    @pytest.mark.asyncio
+    async def test_validate_key_success(self, mock_db):
+        """Test validating a valid API key."""
+        with patch("app.services.auth.db_connection", mock_db):
+            result = await AuthService.validate_key("test-api-key")
+            assert result is not None
+            assert result.key == "test-api-key"
+            assert result.name == "test-client"
+            assert result.is_active in (True, 1)  # SQLite returns 1 for boolean true
+
+    @pytest.mark.asyncio
+    async def test_validate_key_invalid(self, mock_db):
+        """Test validating an invalid API key."""
+        with patch("app.services.auth.db_connection", mock_db):
+            result = await AuthService.validate_key("invalid-key")
+            assert result is None
+
+    @pytest.mark.asyncio
+    async def test_validate_key_disabled(self, mock_db):
+        """Test validating a disabled API key."""
+        with patch("app.services.auth.db_connection", mock_db):
+            result = await AuthService.validate_key("disabled-key")
+            assert result is None  # Disabled keys should not validate
+
+    @pytest.mark.asyncio
+    async def test_validate_key_returns_client_key_instance(self, mock_db):
+        """Test validate_key returns ClientKey instance."""
+        with patch("app.services.auth.db_connection", mock_db):
+            result = await AuthService.validate_key("test-api-key")
+            assert isinstance(result, ClientKey)
+
+    @pytest.mark.asyncio
+    async def test_validate_key_case_sensitive(self, mock_db):
+        """Test key validation is case sensitive."""
+        with patch("app.services.auth.db_connection", mock_db):
+            result = await AuthService.validate_key("TEST-API-KEY")
+            assert result is None  # Should not match due to case