LPM-140 The Sprinting Update
Adds sprint support across the web UI (board + issue detail), TUI kanban, CLI/REPL, and the Beads-backed storage layer.
Changes:
Introduces sprint entities in storage and exposes sprint operations via IssueService.
Updates web board to support backlog vs. sprint columns, sprint selection, and sprint creation routes.
Updates TUI kanban to show backlog + sprint columns and adds sprint selection/creation actions.
