fix(immich): prevent OOM crash on large images and fix video asset handling

[iAndrea]

Problem

The Immich plugin was downloading full-resolution originals (up to 24MB+) on
every refresh, causing OOM kills on low-RAM devices (e.g. Pi Zero). Additionally,
video assets in albums could be selected and passed to Image.open(), causing
a crash, and a missing import requests caused a NameError that swallowed
the real error in the image loader.

Changes

image_album.py

• Download preview instead of original — replace the hard-coded
  /api/assets/{id}/original URL with a version-aware resolver that probes
  the server once (streaming GET, no body downloaded) and caches the working
  endpoint for the lifetime of the provider:
  1. /api/assets/{id}/thumbnail?size=preview — current Immich API
  2. /api/assets/{id}/thumbnail — older versions without size param
  3. Falls back to /original with a warning if both fail
• Switch asset fetch to album endpoint — replace the paginated
  POST /api/search/metadata loop with a single GET /api/albums/{id},
  with client-side type == "IMAGE" filtering to exclude video assets
• README — add asset.view to the required API key permissions

image_loader.py

• Fix NameError in exception handlers — requests was referenced in
  except requests.exceptions.RequestException but never imported
• Eliminate redundant memory copy in _load_from_url_fast — replace
  BytesIO(response.content) (two copies of compressed bytes in memory) with
  response.raw piped directly into PIL; decode_content=True handles
  gzip/deflate transparently

Required API key permissions

asset.view (NEW), asset.read, asset.download, album.read