Centralize Stellar amount unit conversions

[silentgeckoaudit3801]

Closes #222.

Changes:

• adds contract_tools/amounts.py with typed from_stellar_units() and to_stellar_units() helpers
• centralizes raw-unit conversion in deposit transaction data, extra-deposit preparation, and health-ratio debt conversion
• documents the rounding policy with focused helper tests
• adds static wiring coverage so scattered Decimal(...) * 10 ** decimals conversions do not return

Validation:

• parsed changed Python files with ast.parse
• did not run the full project test suite locally because that would require installing/executing project dependencies in this environment