feat(external): per-host enable toggles in Settings (Phase 4d)

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>

alembic/versions/0050_external_link_host_toggles.py

@@ -0,0 +1,38 @@
+"""import_settings: per-host enable toggles for external file-host downloads
+
+Operator levers (#830): disable a single host (e.g. mega.nz when it's
+rate-limiting/banning) without touching the others. The worker reads these via
+getattr and defaults to enabled, so the toggles default TRUE (works out of the
+box, rule #26).
+
+Revision ID: 0050
+Revises: 0049
+Create Date: 2026-06-14
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0050"
+down_revision: Union[str, None] = "0049"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+_HOSTS = ("mega", "gdrive", "mediafire", "dropbox", "pixeldrain")
+
+
+def upgrade() -> None:
+    for host in _HOSTS:
+        op.add_column(
+            "import_settings",
+            sa.Column(
+                f"extdl_{host}_enabled", sa.Boolean(), nullable=False,
+                server_default=sa.true(),
+            ),
+        )
+
+
+def downgrade() -> None:
+    for host in _HOSTS:
+        op.drop_column("import_settings", f"extdl_{host}_enabled")