feat: add board backgrounds

[bionicman]

Description

Adds optional solid-colour and uploaded-image backgrounds to boards.

I built this for a sizeable Trello migration where board backgrounds are useful
visual landmarks across a large workspace. This follows the direction started
in #513 and extends it with image storage, dashboard and public rendering,
copy/template behaviour, and import support. I would coordinate the final PR
shape with @knom rather than silently replacing that work.

• choose from Kan's existing colour palette or upload a background image;
• preserve a valid imported hex colour without adding it to the manual palette;
• render the same background on authenticated boards, public boards and board
  tiles while retaining the current patterned fallback for undecorated boards;
• generate bounded 480/960/1920 WebP previews and lazily resolve only the visible
  board images;
• copy board-owned image objects when creating a board from a board or template;
• import Trello colour and image backgrounds through the built-in adapter,
  materialising remote images into Kan-controlled object storage;
• use the existing board:edit permission and board menu/modal patterns.

The schema remains board-owned (backgroundColourCode and
backgroundImageKey), and API responses expose a discriminated background value
rather than storage keys or internal IDs. Replacement, removal, failed uploads
and failed copies have explicit object-cleanup paths.

I realise that decoration features have a noticeable effect on the interface
and may deserve a wider product discussion. The integration is deliberately
non-invasive: boards without a selected background retain Kan's current
patterned appearance, while teams moving from Trello can preserve the visual
landmarks they already use. The manual picker stays within Kan's existing
palette. I am open to constructive suggestions on the UX and boundaries.

This PR is intended to be considered together with the card-cover work in #605.
They are separate review units, but solve the same navigation and
personalisation problem on boards and cards respectively.

Screenshots

Board gallery:

Decorated board:

Background settings:

Anonymous public board:

Mobile gallery:

Verification

• branch refreshed onto current upstream/main
• targeted API tests: 75 passed
• targeted web tests: 3 passed
• repository integration tests: 5 passed on isolated PGlite databases
• API, DB and shared typechecks
• web typecheck (blocked only by the two existing bootstrap.cjs errors;
  the branch removes the two board visibility errors present on upstream/main)
• entirely new source/test files pass package ESLint and Prettier checks
• self-hosted Playwright background scenario
• synthetic dark-theme desktop, mobile, settings and public screenshots
• screenshot and source-asset checksums verified

Type of change

• Bug fix
• Feature (requires an approved issue — see below)
• Refactor / chore
• Documentation

Checklist

• I have linked the related issue below
• My code follows the existing style and conventions
• I have tested my changes locally
• I have included screenshots for any UI changes

Linked issue

Part of #155