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

redmine_reporter/yaml_config.py

@@ -0,0 +1,58 @@
+import logging
+import os
+import re
+from pathlib import Path
+
+logger = logging.getLogger(__name__)
+
+_ENV_VAR_RE = re.compile(r"\$\{([^}]+)\}")
+
+
+def resolve_env_vars(value: str) -> str:
+    """Replace ${VAR} patterns with environment variable values.
+
+    Non-recursive: if ${A} expands to a literal ${B}, ${B} is NOT resolved.
+    Unknown variables are replaced with empty string and a warning is logged.
+    """
+    if not isinstance(value, str):
+        return value
+
+    def _replacer(match: re.Match) -> str:
+        var_name = match.group(1)
+        env_value = os.environ.get(var_name)
+        if env_value is None:
+            logger.warning("Environment variable %s is not set, using empty string", var_name)
+            return ""
+        return env_value
+
+    return _ENV_VAR_RE.sub(_replacer, value)
+
+
+def ensure_config_dir(path: Path | None = None) -> Path:
+    """Create ~/.config/redmine-reporter/ with 0o700 permissions.
+
+    Returns the path to the config directory.
+    No-op if the directory already exists.
+    """
+    if path is None:
+        path = Path.home() / ".config" / "redmine-reporter"
+    path.mkdir(mode=0o700, parents=True, exist_ok=True)
+    return path
+
+
+def check_file_permissions(path: Path) -> list[str]:
+    """Check that a config file has safe permissions (0o600 or stricter).
+
+    Returns a list of warning messages. Empty list means no issues.
+    """
+    warnings: list[str] = []
+    if not path.exists():
+        return warnings
+
+    mode = path.stat().st_mode & 0o777
+    if mode > 0o600:
+        warnings.append(
+            f"Config file permissions are too open ({mode:04o}). "
+            f"Expected 0600. Fix with: chmod 600 {path}"
+        )
+    return warnings