Persist display content and queued uploads

[LordMike]

This PR persists OpenDisplay display content and the latest queued image delivery across Home Assistant restarts.

Previously, queued image delivery was runtime-only. If HA restarted while a sleepy display had a pending image, the queue slot was lost and the content/pending entities no longer represented the intended delivery state. This makes the delivery queue behave more like an actual queue: once HA accepts a queued image, it can survive a restart and resume waiting for the next device wake.

Changes:

• Adds a versioned HA Store for persisted display content and queued upload state.
• Persists the latest queued prepared image, preview JPEG, refresh mode, PartialState, target device, queue timestamps, attempts, and paused/auth state.
• Restores a pending queued upload on config entry setup if it has not expired.
• Rearms the queue expiry timer using the original expires_at timestamp rather than starting a fresh timeout after restart.
• Avoids storing large image payloads in config entry data; persisted content goes through HA storage instead.
• Delays image-bearing persistence writes for 60 seconds so quickly drained queues are usually not written, while queue-clearing writes use a 1 second delay so delivered content is not resurrected after restart.

Entities with persistence:

• Content image entity:
  • Persists the last JPEG returned by the image entity.
  • Persists its existing queue-related attributes: pending, queued_at, and last_error.
• Update pending binary sensor:
  • Restores its state from the persisted delivery queue snapshot through the delivery manager.
  • Restores queued_at, expires_at, attempts, and last_error attributes.
• Last seen sensor:
  • Restores the last timestamp through HA’s native RestoreEntity path.
  • Stays available after a timestamp has been observed.

Validation:

• pytest -q -p no:cacheprovider tests
• compileall for the integration and updated tests