feat: add encrypted sync user management

Adds full CRUD for sync users (create/list/detail/edit) behind admin
auth and CSRF protection. Passwords are encrypted at rest and never
decrypted into a template context; only emails may be decrypted for
display on the edit form. Blank password fields on edit preserve the
existing encrypted password.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/web/forms.py

@@ -0,0 +1,11 @@
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class UserFormData:
+    name: str
+    mywhoosh_email: str
+    mywhoosh_password: str
+    garmin_email: str
+    garmin_password: str
+    enabled: bool