Add user locale preference support and locale detection logic

- Introduced `locale` field in user model and schemas with BCP 47 format validation.
- Created Alembic migration to add `locale` column to the `users` table with indexing for better query performance.
- Implemented `get_locale` dependency to detect locale using user preference, `Accept-Language` header, or default to English.
- Added extensive tests for locale validation, dependency logic, and fallback handling.
- Enhanced documentation and comments detailing the locale detection workflow and SUPPORTED_LOCALES configuration.

backend/app/models/user.py

@@ -16,6 +16,7 @@ class User(Base, UUIDMixin, TimestampMixin):
     is_active = Column(Boolean, default=True, nullable=False, index=True)
     is_superuser = Column(Boolean, default=False, nullable=False, index=True)
     preferences = Column(JSONB)
+    locale = Column(String(10), nullable=True, index=True)
     deleted_at = Column(DateTime(timezone=True), nullable=True, index=True)
 
     # Relationships