Operator lever: disable a single file host (e.g. mega.nz when it's banning)
without touching the others. Five booleans on import_settings
(extdl_<host>_enabled, default true — works out of the box, rule #26); the
worker already reads them via getattr so no worker change. Migration 0050 +
model fields + settings GET/PATCH (uniform boolean validation) + a
'External file-host downloads' card in the subscriptions Settings tab.
Completes Phase 4. Refs FC #830.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
