Allow to authenticate with the same account from different sources

A new config allows to treat an account same independent where it was
created (openid or local).

Issue: #1827 #1781

modules/restserver/src/main/resources/reference.conf

@@ -95,6 +95,17 @@ docspell.server {
       # How long the remember me cookie/token is valid.
       valid = "30 days"
     }
+
+    # One of: fail, convert
+    #
+    # Accounts can be local or defined at a remote provider and
+    # integrated via OIDC. If the same account is defined in both
+    # sources, docspell by default fails if a user mixes logins (e.g.
+    # when registering a user locally and then logging in with the
+    # same user via OIDC). When set to `convert` docspell treats it as
+    # being the same and simply updates the account to reflect the new
+    # account source.
+    on-account-source-conflict = "fail"
   }
 
   # Settings for "download as zip"