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>

backend/app/models/import_settings.py

@@ -73,6 +73,25 @@ class ImportSettings(Base):
         Float, nullable=False, default=0.5,
     )
 
+    # #830 off-platform file-host downloads — per-host enable lever (default on,
+    # rule #26). Column names are extdl_<host>_enabled so the worker reads them
+    # via getattr(settings, f"extdl_{host}_enabled", True).
+    extdl_mega_enabled: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=True, server_default="true",
+    )
+    extdl_gdrive_enabled: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=True, server_default="true",
+    )
+    extdl_mediafire_enabled: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=True, server_default="true",
+    )
+    extdl_dropbox_enabled: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=True, server_default="true",
+    )
+    extdl_pixeldrain_enabled: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=True, server_default="true",
+    )
+
     @classmethod
     async def load(cls, session) -> ImportSettings:
         """The singleton settings row (id=1), via an async session."""