feat: finish the Discord switchover — recapture on every native source, backfills that run, gallery-dl's Discord config retired (milestone 428)

- Recover and Recapture show on every native source. The menu gated them on
  a copied platform list ('patreon', 'subscribestar') that went stale when
  Discord moved over. Sources now carry `native_ingester` from the backend's
  own predicate.
- A running backfill is due on every scheduler tick. Nothing queued a
  backfill's next chunk: each one waited for the source's regular interval,
  so an armed backfill sat idle until the next check (8h at the default) and
  a five-chunk walk took most of two days. The in-flight guard and the
  platform lock keep one chunk at a time. A failing source falls back to its
  backoff, and a stalled or out-of-budget walk stops being due. It also runs
  when the artist has auto-check off, since the operator started it by hand.
- gallery-dl no longer carries Discord: its naming constants, platform
  defaults, sidecar-mirroring postprocessor and token injection are gone.
  The naming test moves to the native downloader and still renders against
  the real gallery-dl sidecar fixture. That is the guard that the files
  gallery-dl wrote are found on disk rather than fetched again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

frontend/src/components/subscriptions/SourceActions.vue

@@ -76,10 +76,10 @@ const running = computed(() => props.source.backfill_state === 'running')
 const recovering = computed(() => !!props.source.backfill_bypass_seen)
 const recapturing = computed(() => !!props.source.backfill_recapture)
 // Recover / recapture are native-ingester features (ledger-bypass re-walk and
-// post-text re-grab), available to every native platform — not just Patreon.
-// Mirrors backend download_backends.NATIVE_INGESTER_PLATFORMS.
-const NATIVE_PLATFORMS = ['patreon', 'subscribestar']
-const isNative = computed(() => NATIVE_PLATFORMS.includes(props.source.platform))
+// post-text re-grab), available on every native platform. The backend says
+// which those are (`native_ingester`); a copied list here went stale when
+// Discord moved over.
+const isNative = computed(() => !!props.source.native_ingester)
 </script>
 
 <style scoped>

frontend/src/components/subscriptions/SubscriptionsTab.vue

@@ -679,7 +679,7 @@ async function onRecover(source) {
   }
 }
 
-// #830: arm a recapture walk (Patreon-only) — re-grab every post's body +
+// #830: arm a recapture walk (native platforms) — re-grab every post's body +
 // external links and localize on-disk inline images, without re-downloading
 // media. Reuses the backfill lifecycle/badge; stop via the same Stop control.
 async function onRecapture(source) {

frontend/src/stores/sources.js

@@ -140,7 +140,7 @@ export const useSourcesStore = defineStore('sources', () => {
     _patchSource(body)
     return body
   }
-  // #830: arm a recapture walk (Patreon-only) — re-grab every post's body +
+  // #830: arm a recapture walk (native platforms) — re-grab every post's body +
   // external links and localize on-disk inline images, WITHOUT re-downloading
   // media. Shares the backfill lifecycle/badge; stop via stopBackfill.
   async function recaptureSource(id, artistIdHint = null) {