feat: auto-download missing queue tracks and add library play actions

[zirkyspeng]

Summary

This PR aims to add 2 new related features:

• feat(playback): auto-download missing queue tracks
  • Automatically acquire missing playback-queue tracks through SoulSync's existing enabled download sources.
  • Preserve queue order with one active queue download so the next unavailable track is prepared first.
  • Reconcile completed downloads with their imported library files so queue entries become playable instead of remaining Missing.
• feat(library): add artist and album play actions
  • Add Play actions for artists in Library that adds top tracks to the player's queue (and download missing tracks with auto-download enabled)
  • For albums on artist pages, add a play button that adds all tracks to the queue and start playing the albums (and download missing tracks with auto-download enabled)

Behavior

• Queue auto-download is disabled by default and stored per profile.
• Existing library files and active or completed queue downloads are reused.
• Missing tracks expose queued, searching, downloading, verifying, and error states.
• The first unavailable queue track waits for its verified import while later missing tracks remain ordered behind it.
• Downloads use SoulSync's existing source-selection, matching, download, verification, and import pipeline.
• Artist Play queues the artist's top tracks; album Play queues its tracks in album order.

Testing

• python -m ruff check --output-format=github .
• python -m compileall api core database services scripts web_server.py wsgi.py beatport_unified_scraper.py
• Targeted Python tests: 103 passed.
• Full Python suite: 15,133 passed; three unrelated failures were reproduced unchanged on dev.
• Targeted WebUI tests: 153 passed.
• Full WebUI suite: 8,032 passed.
• WebUI production build completed successfully.
• Changed WebUI sources pass formatting and type-aware lint with no errors.
• The repository-wide WebUI format check is already failing on dev (36 pre-existing files); this branch reduces that count and introduces no formatting failure in its changed sources.

Screenshots

Library artist Play action

Artist-page album Play action

Ordered queue auto-download states