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>

frontend/src/components/subscriptions/SettingsTab.vue

@@ -52,6 +52,52 @@
       </v-card-text>
     </v-card>
 
+    <h3 class="text-h6 mb-3 mt-6">External file-host downloads</h3>
+    <v-card variant="outlined">
+      <v-card-text v-if="importStore.settings">
+        <div class="fc-help mb-2">
+          When a creator links files on these hosts in a post, Curator downloads
+          them into the post automatically (tagged + provenance-linked like any
+          other media). Turn a host off if it starts rate-limiting or banning.
+        </div>
+        <v-row>
+          <v-col cols="12" sm="4">
+            <v-switch
+              v-model="dl.extdl_mega_enabled" label="mega.nz"
+              density="compact" hide-details color="accent" @change="saveDownloader"
+            />
+          </v-col>
+          <v-col cols="12" sm="4">
+            <v-switch
+              v-model="dl.extdl_gdrive_enabled" label="Google Drive"
+              density="compact" hide-details color="accent" @change="saveDownloader"
+            />
+          </v-col>
+          <v-col cols="12" sm="4">
+            <v-switch
+              v-model="dl.extdl_pixeldrain_enabled" label="Pixeldrain"
+              density="compact" hide-details color="accent" @change="saveDownloader"
+            />
+          </v-col>
+          <v-col cols="12" sm="4">
+            <v-switch
+              v-model="dl.extdl_mediafire_enabled" label="MediaFire"
+              density="compact" hide-details color="accent" @change="saveDownloader"
+            />
+          </v-col>
+          <v-col cols="12" sm="4">
+            <v-switch
+              v-model="dl.extdl_dropbox_enabled" label="Dropbox"
+              density="compact" hide-details color="accent" @change="saveDownloader"
+            />
+          </v-col>
+        </v-row>
+      </v-card-text>
+      <v-card-text v-else>
+        <v-skeleton-loader type="paragraph" />
+      </v-card-text>
+    </v-card>
+
     <h3 class="text-h6 mb-3 mt-6">Schedule defaults</h3>
     <v-card variant="outlined">
       <v-card-text v-if="importStore.settings">
@@ -151,6 +197,11 @@ const dl = reactive({
   download_schedule_default_seconds: 28800,
   download_event_retention_days: 90,
   download_failure_warning_threshold: 5,
+  extdl_mega_enabled: true,
+  extdl_gdrive_enabled: true,
+  extdl_mediafire_enabled: true,
+  extdl_dropbox_enabled: true,
+  extdl_pixeldrain_enabled: true,
 })
 
 watch(() => importStore.settings, (s) => { if (s) Object.assign(dl, s) }, { immediate: true })