Drop the unused project description field entirely

It never got a UI home on the frontend -- removed from ProjectView a
few sessions back and never restored anywhere -- so it was a fully
live field (validated, stored, returned by the API, covered by tests)
with no consumer.

- Migration 011: ALTER TABLE projects DROP COLUMN description.
- ProjectRepository: description dropped from ProjectRow, SELECT,
  create() and update() -- update() is now just a rename (string, not
  a $fields array; there was never more than one editable field once
  this left).
- ProjectController: store()/update() no longer accept or validate
  it; update() drops the "provide at least one of title, description"
  branch, since title is unconditionally the only field now.
- Frontend Project type, root README's API docs and curl example.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/types.ts

@@ -26,7 +26,6 @@ export interface Passkey {
 export interface Project {
   id: number
   title: string
-  description: string
   owner_id: number
   card_count: number
   completed_count: number