Improve branch, merge, rebase (#37)

* Always create task branch

* Create new ref

* Save new branch name in DB

* Refactor rebase backend

* Update merging functionality

* Clippy

backend/migrations/20250701000000_add_branch_to_task_attempts.sql

@@ -0,0 +1,2 @@
+-- Add branch column to task_attempts table
+ALTER TABLE task_attempts ADD COLUMN branch TEXT NOT NULL DEFAULT '';