feat: YAML config support (~/.config/redmine-reporter/config.yml)

- Add AppConfig/SmtpConfig/EmailConfig dataclasses with from_yaml()/from_env()
- Add yaml_config.py: ${VAR} resolver, 0700/0600 permission helpers
- Config.get_*() methods gain YAML fallback in priority chain
- Priority: CLI > env > .env > YAML > code defaults
- CLI --init-config generates YAML from current environment
- --force flag allows overwriting existing config
- Secrets default to ${VAR} references, plaintext allowed
- Full backward compatibility with existing .env setups

Closes #46

pyproject.toml

@@ -24,6 +24,7 @@ dependencies = [
     "python-dotenv>=1.0.0",
     "odfpy>=1.4.0",
     "openpyxl>=3.1.0",
+    "pyyaml>=6.0",
 ]
 
 [project.optional-dependencies]