serve: `POST /tasks` create endpoint (lands in backlog)

[andreiRS]

Part of the serve-board spec (docs/specs/serve-board.md), architecture in docs/adr/0016-board-server-reuses-core-in-process.md.

Effort

S — one endpoint reusing createTask.

What to build

POST /api/tasks that creates a Task via the existing createTask core (title required, optional body incl. acceptance criteria, effort low/med/high). Lands in backlog, attendance defaults to attended, exactly like tasks new. One commit.

Acceptance criteria

• A create lands a new Task file in backlog/ with a freshly allocated Short ID and the given effort, producing one commit.
• Missing/empty title is rejected with the correct error code.
• The new task appears in the next GET /api/board snapshot.

Blocked by

#13

[andreiRS]

Done on feat/serve-board, commit b946910. POST /api/tasks reuses createTask: body {title, body?, effort?} (effort default medium), lands in backlog/attended, success 201 {ok,id,uuid,column}; missing title → INVALID_TITLE, bad effort → INVALID_EFFORT. tsc clean, full suite 364 pass. (A type error in the includes-narrowing was caught in verification and folded into the slice commit.)