feat(pixiv): flip dispatch to the native ingester (#129 step 4)

pixiv joins NATIVE_INGESTER_PLATFORMS: download/verify/preview and the
recover/recapture UI actions now route through PixivIngester. Campaign id is
parsed straight from the source URL (numeric user id — no network resolver),
with a platform-aware resolution-failure message. auth_token now rides the
uniform adapter construction (token platforms use it, cookie platforms
accept-and-ignore), and the preview endpoint fetches/threads it. The legacy
gallery-dl pixiv path is fully removed (PLATFORM_DEFAULTS entry + the
refresh-token config branches in download/verify) per no-legacy policy;
gallery-dl keeps hentaifoundry/discord/deviantart until they migrate/retire.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

frontend/src/components/subscriptions/SourceActions.vue

@@ -76,7 +76,8 @@ 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.
-const NATIVE_PLATFORMS = ['patreon', 'subscribestar']
+// Mirrors backend download_backends.NATIVE_INGESTER_PLATFORMS.
+const NATIVE_PLATFORMS = ['patreon', 'subscribestar', 'pixiv']
 const isNative = computed(() => NATIVE_PLATFORMS.includes(props.source.platform))
 </script>