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/api/settings.py

@@ -27,6 +27,20 @@ _EDITABLE_FIELDS = (
     "download_failure_warning_threshold",
     "series_suggest_enabled",
     "series_suggest_threshold",
+    "extdl_mega_enabled",
+    "extdl_gdrive_enabled",
+    "extdl_mediafire_enabled",
+    "extdl_dropbox_enabled",
+    "extdl_pixeldrain_enabled",
+)
+
+# Per-host external-download toggles — all plain booleans, validated uniformly.
+_EXTDL_TOGGLE_FIELDS = (
+    "extdl_mega_enabled",
+    "extdl_gdrive_enabled",
+    "extdl_mediafire_enabled",
+    "extdl_dropbox_enabled",
+    "extdl_pixeldrain_enabled",
 )
 
 
@@ -50,6 +64,11 @@ async def get_import_settings():
         "download_failure_warning_threshold": row.download_failure_warning_threshold,
         "series_suggest_enabled": row.series_suggest_enabled,
         "series_suggest_threshold": row.series_suggest_threshold,
+        "extdl_mega_enabled": row.extdl_mega_enabled,
+        "extdl_gdrive_enabled": row.extdl_gdrive_enabled,
+        "extdl_mediafire_enabled": row.extdl_mediafire_enabled,
+        "extdl_dropbox_enabled": row.extdl_dropbox_enabled,
+        "extdl_pixeldrain_enabled": row.extdl_pixeldrain_enabled,
     })
 
 
@@ -106,6 +125,9 @@ async def update_import_settings():
         return jsonify(
             {"error": "series_suggest_enabled must be a boolean"}
         ), 400
+    for tog in _EXTDL_TOGGLE_FIELDS:
+        if tog in body and not isinstance(body[tog], bool):
+            return jsonify({"error": f"{tog} must be a boolean"}), 400
     if "series_suggest_threshold" in body:
         v = body["series_suggest_threshold"]
         if not isinstance(v, (int, float)) or isinstance(v, bool) or v < 0 or v > 1: