fix: gate LocalUser machinery on multi_user_enabled for single-user backward compat

- auth() only queries LocalUser table when multi_user_enabled=True
- login() only shows signup link when multi_user_enabled AND allow_local_signup
- signup page and POST endpoint both check multi_user_enabled first
- Move local-auth imports to module level in auth.py (no re-import overhead)
- Fix signup rollback: flush before email send, commit only on success
- Update allow_local_signup config description to document prerequisite
- Add test: single-user mode skips LocalUser table entirely
- Patch multi_user_enabled=True on all local-login integration tests

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/config.py

@@ -177,8 +177,9 @@ class Settings(BaseSettings):
         default=False,
         description=(
             "Allow users to self-register with email and password. "
-            "Requires email (SMTP) to be configured for verification emails. "
-            "Default: False (registration disabled, admin creates users)."
+            "Has no effect unless MULTI_USER_ENABLED is also True. "
+            "Requires SMTP to be configured so verification emails can be sent. "
+            "Default: False (registration disabled — admin creates users manually)."
         ),
     )