The custom API key authentication only verified that the APIToken row was
active and unexpired; it never checked the owning user's is_active flag.
DRF's IsAuthenticated only checks user.is_authenticated (always True for a
real User), so a user whose account was deactivated could keep using a
previously issued API key indefinitely.
Add user__is_active=True to the validate_api_token() lookup so a token tied
to a disabled account is treated as invalid (a generic AuthenticationFailed,
avoiding account-state disclosure). Applied to both the external API
middleware (plane/api) and the identical, currently unused copy in
plane/app to prevent the gap from being reintroduced.
Adds unit coverage on validate_api_token and an end-to-end contract test
proving GET /api/v1/users/me/ is denied once the account is deactivated.
