Add parallel checker execution with connection pooling

Implements Level 2 parallelization for row_count, schema, and
aggregate checkers, improving performance by 2-3x for tables with
multiple enabled checks.

Changes:
- Add max_workers config option (default: 4)
- Add ConnectionPool module with SQLAlchemy QueuePool
- Add URL encoding for connection strings
- Implement parallel checker execution with ThreadPoolExecutor
- Add fail-fast behavior on checker errors
- Update executor for SQLAlchemy 2.0 compatibility
- Fix engine disposal resource leak
- Cache pooled engines in ConnectionManager
- Add disconnect() cleanup for pooled engines

Performance:
- Sequential: 3 checkers × 100ms = 300ms
- Parallel: 3 checkers ≈ 100ms (2-3x speedup)

Configuration:
  execution:
    max_workers: 4  # Controls parallel checker execution
    continue_on_error: true

tests/test_parallelization.py

@@ -0,0 +1,88 @@
+"""Test parallelization features."""
+
+import pytest
+from drt.config.models import Config, ExecutionConfig
+
+
+class TestParallelizationConfig:
+    """Test parallelization configuration."""
+    
+    def test_default_max_workers(self):
+        """Test default max_workers value."""
+        config = Config()
+        assert config.execution.max_workers == 4
+    
+    def test_custom_max_workers(self):
+        """Test custom max_workers value."""
+        config = Config(execution=ExecutionConfig(max_workers=8))
+        assert config.execution.max_workers == 8
+    
+    def test_max_workers_positive(self):
+        """Test that max_workers is positive."""
+        config = Config()
+        assert config.execution.max_workers > 0
+    
+    def test_continue_on_error_default(self):
+        """Test default continue_on_error value."""
+        config = Config()
+        assert config.execution.continue_on_error is True
+
+
+def test_imports():
+    """Test that all parallelization modules can be imported."""
+    from drt.config.models import Config, ExecutionConfig
+    from urllib.parse import quote_plus
+    from sqlalchemy import create_engine, text, QueuePool
+    
+    assert Config is not None
+    assert ExecutionConfig is not None
+    assert quote_plus is not None
+    assert create_engine is not None
+    assert text is not None
+    assert QueuePool is not None
+
+
+def test_url_encoding():
+    """Test URL encoding for connection strings."""
+    from urllib.parse import quote_plus
+    
+    test_conn_str = "DRIVER={ODBC Driver};SERVER=localhost;PWD=test@pass#123"
+    encoded = quote_plus(test_conn_str)
+    
+    # Special characters should be encoded
+    assert "%40" in encoded  # @ encoded
+    assert "%23" in encoded  # # encoded
+    assert "%3D" in encoded  # = encoded
+    assert encoded != test_conn_str  # Should be different after encoding
+    
+    # Test that it can be decoded back
+    from urllib.parse import unquote_plus
+    decoded = unquote_plus(encoded)
+    assert decoded == test_conn_str
+
+
+def test_config_load_minimal():
+    """Test loading a minimal config with parallelization settings."""
+    config_dict = {
+        "database_pairs": [
+            {
+                "name": "Test",
+                "enabled": True,
+                "baseline": {"server": "S1", "database": "D1"},
+                "target": {"server": "S1", "database": "D2"}
+            }
+        ],
+        "execution": {
+            "max_workers": 6,
+            "continue_on_error": False
+        },
+        "tables": []
+    }
+    
+    config = Config(**config_dict)
+    assert config.execution.max_workers == 6
+    assert config.execution.continue_on_error is False
+
+
+if __name__ == "__main__":
+    pytest.main([__file__, "-v"])