fix(seed): don't create a duplicate admin on restart after email migration

seed_admin()'s existence check matched on a freshly-computed email/username
identifier — after the email-unique-identifier migration backfills an
older install's admin email to its bare username (e.g. 'administrator'),
that no longer matches the '...@localhost' placeholder computed when
ADMIN_EMAIL isn't set, so every container restart created a new duplicate
admin. Now skips seeding whenever ANY admin role already exists.

backend/app/seed.py

@@ -64,12 +64,18 @@ def seed_admin() -> None:
         if not admin_email:
             admin_email = admin_username if "@" in admin_username else f"{admin_username}@localhost"
 
-        # Assign existing = db.query(User).filter(User.email == admin_email).first()
-        existing = db.query(User).filter(User.email == admin_email).first()
+        # Skip if ANY admin already exists — not just one matching this
+        # specific username/email. Matching only on the computed identifier
+        # is fragile: an install predating ADMIN_EMAIL has a user whose
+        # email was migration-backfilled to its bare username (e.g.
+        # "administrator"), which won't match a freshly-computed
+        # "...@localhost" placeholder, so every restart would otherwise
+        # create a new duplicate admin account.
+        existing = db.query(User).filter(User.role == "admin").first()
         # Check: existing
         if existing:
             # Call print()
-            print(f"Admin user '{admin_email}' already exists — skipping.")
+            print(f"An admin user already exists ('{existing.email}') — skipping.")
             # Return control to caller
             return