Fix unit tests for setuptools v82 compatibility

[Copilot]

Setuptools v82 removed the deprecated pkg_resources module, breaking passlib==1.7.4 which depends on it.

Changes

• Replaced passlib with secrets module: The only usage was passlib.pwd.genword(length=10) for GPG key password generation in src/admin_gpg.py. Replaced with Python's built-in secrets.choice() over an alphanumeric alphabet.
• Removed passlib dependency from requirements.txt
• Extracted _PASSWORD_ALPHABET constant at module level for maintainability

# Before
from passlib.pwd import genword
password = genword(length=10)

# After
import secrets
import string

_PASSWORD_ALPHABET = string.ascii_letters + string.digits
password = "".join(secrets.choice(_PASSWORD_ALPHABET) for _ in range(10))

The secrets module provides cryptographically strong random generation suitable for passwords and tokens.

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