feat: config model and yaml loader

Add Pydantic config models (SqlServerConfig, AccessConfig, RuntimeConfig,
FileMapping, SyncConfig) and a YAML loader (load_config). FileMapping provides
source_path() and target_table() helpers; number-typed YAML keys/values (e.g.
root: 2026) are coerced to str via coerce_numbers_to_str. Includes
config.example.yaml template (config.yaml with real credentials stays
gitignored) and pyproject.toml pytest config (pythonpath=src).

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

pyproject.toml

@@ -0,0 +1,4 @@
+[tool.pytest.ini_options]
+pythonpath = ["src"]
+testpaths = ["tests"]
+markers = ["integration: marks tests requiring real Access/SQL Server"]