feat(subs-hub): collapse /credentials + /downloads into /subscriptions hub with three GS-style subtabs

Replaces the three top-level routes with a single `/subscriptions` parent
owning the whole download-pipeline domain. Internal tab state via `?tab=`
query param, mirroring ArtistView's pattern. TopNav auto-drops the two
removed entries (route-driven via meta.title). Bookmark-safe redirects
from `/credentials` and `/downloads` route into the appropriate subtab.

**Subtab 1 — Subscriptions (default).** Carries over the existing
artist-grouped expandable table; adds (a) status filter dropdown, (b)
bulk-select column with Enable/Disable/Delete-all actions, (c) GS-style
color-coded `PlatformChip` per distinct platform in the collapsed row.
Reuses SourceRow, SourceHealthDot, SourceFormDialog, ArtistCreateDialog.

**Subtab 2 — Downloads.** Full GS dashboard. Five colored stat chips up
top (Queued/Running/Completed/Failed/Skipped, sourced from new
`GET /api/downloads/stats?window_hours=`). Popover-style filter UI
(Status/Source/FromDate/ToDate) with active-filter pills below.
Maintenance menu wraps existing /api/import/retry-failed and
/api/import/clear-stuck endpoints; Export-failed-logs item disabled with
a "v2" tooltip. Per-row Retry preserved via existing DownloadEventRow.

**Subtab 3 — Settings.** Four sections: ExtensionKeyBar (top), GS-style
per-platform CredentialCard grid (md=6 v-row/v-col, dashed border if
unset / accent border if set, expandable how-to panel), Downloader card
(rate limit, validate_files), Schedule defaults card (default interval,
event retention, failure warning threshold). The Downloader and Schedule
sections were extracted out of components/settings/ImportFiltersForm.vue
— SettingsView's Import tab now owns only image-import filters.

**Backend:** new `GET /api/downloads/stats` returns
{pending, running, ok, error, skipped} count grouped by status over the
configurable window. Status keys stay raw from the ENUM; UI does the
display-label mapping. Two integration tests pin the response shape +
window_hours validation.

**Util:** `frontend/src/utils/platformColor.js` — single source of truth
for the six platforms' color + icon + label, mirroring GS's palette
(patreon=red mdi-patreon, subscribestar=amber mdi-star,
hentaifoundry=purple mdi-palette, discord=indigo mdi-discord,
pixiv=blue mdi-alpha-p-box, deviantart=green mdi-deviantart). Unknown
platform falls back to grey + mdi-web.

Deferred (explicit non-goals): subscription import/export, "Trigger Due
Now" scheduler-tick button (needs new backend endpoint), Export Failed
Logs CSV dump.

frontend/src/components/subscriptions/MaintenanceMenu.vue

@@ -0,0 +1,62 @@
+<template>
+  <v-menu>
+    <template #activator="{ props }">
+      <v-btn v-bind="props" variant="outlined" prepend-icon="mdi-wrench" append-icon="mdi-chevron-down">
+        Maintenance
+      </v-btn>
+    </template>
+    <v-list density="compact">
+      <v-list-item
+        prepend-icon="mdi-refresh"
+        title="Retry failed"
+        subtitle="Re-enqueue every failed import task"
+        @click="onRetry"
+      />
+      <v-list-item
+        prepend-icon="mdi-broom"
+        title="Clear stuck"
+        subtitle="Mark long-running import tasks failed and finalize their batch"
+        @click="onClear"
+      />
+      <v-list-item
+        :disabled="true"
+        prepend-icon="mdi-download-box"
+        title="Export failed logs"
+        subtitle="CSV dump — v2"
+      >
+        <v-tooltip activator="parent" location="start">Deferred to a future release</v-tooltip>
+      </v-list-item>
+    </v-list>
+  </v-menu>
+</template>
+
+<script setup>
+import { useImportStore } from '../../stores/import.js'
+
+const emit = defineEmits(['refresh'])
+const importStore = useImportStore()
+
+async function onRetry() {
+  try {
+    await importStore.retryFailed()
+    globalThis.window?.__fcToast?.({ text: 'Retry queued', type: 'success' })
+    emit('refresh')
+  } catch (e) {
+    globalThis.window?.__fcToast?.({ text: `Retry failed: ${e?.detail || e?.message || e}`, type: 'error' })
+  }
+}
+
+async function onClear() {
+  try {
+    const body = await importStore.clearStuck()
+    const n = body?.cleared ?? 0
+    globalThis.window?.__fcToast?.({
+      text: n ? `Cleared ${n} stuck task${n === 1 ? '' : 's'}` : 'Nothing stuck',
+      type: 'success',
+    })
+    emit('refresh')
+  } catch (e) {
+    globalThis.window?.__fcToast?.({ text: `Clear failed: ${e?.detail || e?.message || e}`, type: 'error' })
+  }
+}
+</script>