Refactor: Replace global settings singleton with lazy initialization

[shimpeiws]

Summary

In config.py, settings = Settings() is instantiated at module load time. This makes it difficult to override settings during tests (DI-unfriendly).

Proposed Approach

• Replace the module-level settings = Settings() with a get_settings() function that lazily initializes the singleton
• Allow tests to override settings via the function or a reset mechanism
• Update all call sites that reference settings directly

Affected Files

• src/barscan/config.py
• All files that import settings from config.py