perf(issues): replace N+1 queries with batch operations

- Add Repo::find_by_ids() batch query using WHERE id = ANY($1)
- Replace 3 sequential validation loops (repos, labels, assignees)
  with batch queries using ANY($1)
- Replace 3 sequential INSERT loops with single INSERT...SELECT
  FROM unnest() statements
- Extract 7 helper methods: validate_issue_repos,
  validate_issue_labels, validate_issue_assignees,
  validate_issue_milestone, insert_issue_repo_relations,
  insert_issue_label_relations, insert_issue_assignees
- Reduce issue_create() from ~243 lines to ~80 lines

models/repos/repo_queries.rs

@@ -39,6 +39,26 @@ impl Repo {
         .await
     }
 
+    /// Find multiple non-deleted repos by their IDs in a single query.
+    pub async fn find_by_ids(
+        pool: &PgPool,
+        ids: &[Uuid],
+    ) -> Result<Vec<Self>, sqlx::Error> {
+        if ids.is_empty() {
+            return Ok(Vec::new());
+        }
+        sqlx::query_as::<_, Repo>(
+            r#"SELECT id, workspace_id, owner_id, name, description, default_branch, visibility,
+                      status, is_fork, forked_from_repo_id, storage_node_ids,
+                      primary_storage_node_id, storage_path, git_service,
+                      archived_at, created_at, updated_at, deleted_at
+               FROM repo WHERE id = ANY($1) AND deleted_at IS NULL"#,
+        )
+        .bind(ids)
+        .fetch_all(pool)
+        .await
+    }
+
     /// Check if a user is an active member of a repo.
     pub async fn is_member(
         pool: &PgPool,