refactor(subscriptions): remove the dead backfill dry-run preview (#1281)

PreviewDialog.vue was orphaned — nothing mounted it — so the dry-run preview
was unreachable. Rather than wire it up, remove the whole chain: its unique
value (a capped 3-page count of what a backfill would grab) is low, and its
adjacent needs are already covered — auth validation by verify_source_credential,
and actually fetching recent posts by "Check now". Operator decision 2026-07-06.

Removed:
- frontend: PreviewDialog.vue + sources store previewSource()
- backend: POST /api/sources/<id>/preview route, download_backends.preview_source,
  IngestCore.preview() + its now-unused NativeIngestError import
- tests: the 3 ingester preview tests

Nothing else referenced the chain (verified). Shared campaign-resolution and
ledger helpers stay — they're used by run()/verify_source_credential.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

frontend/src/stores/sources.js

@@ -149,12 +149,6 @@ export const useSourcesStore = defineStore('sources', () => {
     return body
   }
 
-  // Plan #708 B4: dry-run preview — count what a backfill WOULD download
-  // (native platforms), without downloading. Read-only, so no cache invalidate.
-  async function previewSource(id) {
-    return await api.post(`/api/sources/${id}/preview`)
-  }
-
   function sourcesByArtistGrouped() {
     // returns [{artist: {id,name,slug}, sources: [...]}, ...]
     const arr = byArtist.value.get(null) ?? []
@@ -185,7 +179,6 @@ export const useSourcesStore = defineStore('sources', () => {
     stopBackfill,
     recoverSource,
     recaptureSource,
-    previewSource,
     findOrCreateArtist, autocompleteArtist, reassign,
     loadScheduleStatus,
     sourcesByArtistGrouped,