Federated user activity following: Isolated model changes (#8078)

This PR is part of https://codeberg.org/forgejo/forgejo/pulls/4767

This should not have an outside impact but bring all model changes needed & bring migrations.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8078
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Michael Jerger <michael.jerger@meissa-gmbh.de>
Co-committed-by: Michael Jerger <michael.jerger@meissa-gmbh.de>

models/user/federated_user_follower.go

@@ -0,0 +1,30 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package user
+
+import "forgejo.org/modules/validation"
+
+type FederatedUserFollower struct {
+	ID              int64 `xorm:"pk autoincr"`
+	FollowedUserID  int64 `xorm:"NOT NULL unique(fuf_rel)"`
+	FollowingUserID int64 `xorm:"NOT NULL unique(fuf_rel)"`
+}
+
+func NewFederatedUserFollower(followedUserID, federatedUserID int64) (FederatedUserFollower, error) {
+	result := FederatedUserFollower{
+		FollowedUserID:  followedUserID,
+		FollowingUserID: federatedUserID,
+	}
+	if valid, err := validation.IsValid(result); !valid {
+		return FederatedUserFollower{}, err
+	}
+	return result, nil
+}
+
+func (user FederatedUserFollower) Validate() []string {
+	var result []string
+	result = append(result, validation.ValidateNotEmpty(user.FollowedUserID, "FollowedUserID")...)
+	result = append(result, validation.ValidateNotEmpty(user.FollowingUserID, "FollowingUserID")...)
+	return result
+}