fix(projects): scope project-issue move to its own project

MoveIssuesOnProjectColumn updated `project_issue` with a WHERE clause on
issue_id only. An issue assigned to several projects has one project_issue
row per project, so moving it within one project rewrote project_board_id
for every project the issue belonged to, detaching it from all the others.

Scope the UPDATE to (issue_id, project_id) so only the target project's
row changes. Mirrors the fix already present in upstream/main.

Adds an integration regression test asserting an issue in two user
projects keeps its column in the other project after a move. Fixes #17.

services/projects/issue.go

@@ -85,7 +85,16 @@ func MoveIssuesOnProjectColumn(ctx context.Context, doer *user_model.User, colum
 				}
 			}
 
-			_, err = db.Exec(ctx, "UPDATE `project_issue` SET project_board_id=?, sorting=? WHERE issue_id=?", column.ID, sorting, issueID)
+			// Scope the update to this issue *in this project*. Without the
+			// project_id predicate, an issue that belongs to several projects
+			// would have every project_issue row rewritten to the target
+			// column, detaching it from all other projects.
+			_, err = db.GetEngine(ctx).Table("project_issue").
+				Where("issue_id = ? AND project_id = ?", issueID, column.ProjectID).
+				Update(map[string]any{
+					"project_board_id": column.ID,
+					"sorting":          sorting,
+				})
 			if err != nil {
 				return err
 			}