refactor: improve language normalization and date handling utilities

- Introduced a new `normalize_lang` function to standardize language codes across the application, ensuring consistent handling of user language preferences.
- Refactored date handling utilities by adding `parse_utc_iso` and `parse_utc_iso_naive` functions for better parsing of ISO 8601 date strings, enhancing timezone awareness.
- Updated various modules to utilize the new language normalization and date parsing functions, improving code clarity and maintainability.
- Enhanced error handling in date validation to raise specific `DateRangeValidationError` exceptions, providing clearer feedback on validation issues.
- Improved test coverage for date range validation and language normalization functionalities, ensuring robustness and reliability.

duty_teller/config.py

@@ -4,13 +4,15 @@ BOT_TOKEN is not validated on import; call require_bot_token() in the entry poin
 when running the bot.
 """
 
-import re
 import os
+import re
 from dataclasses import dataclass
 from pathlib import Path
 
 from dotenv import load_dotenv
 
+from duty_teller.i18n.lang import normalize_lang
+
 load_dotenv()
 
 # Project root (parent of duty_teller package). Used for webapp path, etc.
@@ -34,14 +36,6 @@ def normalize_phone(phone: str | None) -> str:
     return _PHONE_DIGITS_RE.sub("", phone.strip())
 
 
-def _normalize_default_language(value: str) -> str:
-    """Normalize DEFAULT_LANGUAGE from env to 'ru' or 'en'."""
-    if not value:
-        return "en"
-    v = value.strip().lower()
-    return "ru" if v.startswith("ru") else "en"
-
-
 def _parse_phone_list(raw: str) -> set[str]:
     """Parse comma-separated phones into set of normalized (digits-only) strings."""
     result = set()
@@ -113,9 +107,7 @@ class Settings:
             ).strip(),
             duty_display_tz=os.getenv("DUTY_DISPLAY_TZ", "Europe/Moscow").strip()
             or "Europe/Moscow",
-            default_language=_normalize_default_language(
-                os.getenv("DEFAULT_LANGUAGE", "en").strip()
-            ),
+            default_language=normalize_lang(os.getenv("DEFAULT_LANGUAGE", "en")),
         )