Fix password strength validation library (was forcibly disabled)

[Copilot]

The password-strength library was installed and in the dependency spec, but never actually used: the import was placed outside the try/except block, and _PASSWORD_STRENGTH_AVAILABLE was unconditionally overridden to False afterward.

Changes

• backend/common/password_utils.py: Move from password_strength import PasswordPolicy inside the try/except block and remove the hardcoded _PASSWORD_STRENGTH_AVAILABLE = False override

# Before (broken)
from password_strength import PasswordPolicy  # top-level, crashes if missing

try:
    _PASSWORD_STRENGTH_AVAILABLE = True  # try block imports nothing
except ImportError:
    _PASSWORD_STRENGTH_AVAILABLE = False
_PASSWORD_STRENGTH_AVAILABLE = False  # unconditionally disabled

# After (fixed)
try:
    from password_strength import PasswordPolicy
    _PASSWORD_STRENGTH_AVAILABLE = True
except ImportError:
    _PASSWORD_STRENGTH_AVAILABLE = False

When the library is unavailable, falls back to the existing basic regex validation.