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/settings/ImportFiltersForm.vue

@@ -51,68 +51,6 @@
         </v-col>
       </v-row>
 
-      <v-divider class="my-4" />
-      <div class="text-subtitle-2 mb-2">Downloader (FC-3c)</div>
-      <v-row>
-        <v-col cols="12" sm="6">
-          <v-text-field
-            v-model.number="local.download_rate_limit_seconds"
-            label="Rate limit (seconds between requests)"
-            type="number" step="0.5" min="0"
-            density="compact" hide-details @blur="save"
-          />
-          <div class="fc-help">gallery-dl extractor.sleep. Higher = slower but safer.</div>
-        </v-col>
-        <v-col cols="12" sm="6">
-          <v-switch
-            v-model="local.download_validate_files"
-            label="Validate downloaded files (magic-byte check)"
-            density="compact" hide-details color="accent" @change="save"
-          />
-        </v-col>
-      </v-row>
-
-      <v-divider class="my-4" />
-      <div class="text-subtitle-2 mb-2">Download scheduling (FC-3d)</div>
-      <v-row>
-        <v-col cols="12" sm="6">
-          <v-text-field
-            v-model.number="local.download_schedule_default_seconds"
-            label="Default check interval (seconds)"
-            type="number" :min="60" :max="86400"
-            density="compact" hide-details @blur="save"
-          />
-          <div class="fc-help">
-            Used when a source has no per-source or per-artist override.
-            Default 28800 (8 hours).
-          </div>
-        </v-col>
-        <v-col cols="12" sm="6">
-          <v-text-field
-            v-model.number="local.download_event_retention_days"
-            label="Event retention (days)"
-            type="number" :min="1" :max="3650"
-            density="compact" hide-details @blur="save"
-          />
-          <div class="fc-help">
-            Completed download events older than this are deleted nightly.
-            Default 90.
-          </div>
-        </v-col>
-        <v-col cols="12" sm="6">
-          <v-text-field
-            v-model.number="local.download_failure_warning_threshold"
-            label="Failure warning threshold"
-            type="number" :min="1" :max="100"
-            density="compact" hide-details @blur="save"
-          />
-          <div class="fc-help">
-            Source row badge turns red after this many consecutive
-            failures. Sources are never auto-disabled. Default 5.
-          </div>
-        </v-col>
-      </v-row>
-
       <v-alert v-if="store.settingsError" type="error" variant="tonal" class="mt-2" closable>
         {{ store.settingsError }}
       </v-alert>
@@ -128,16 +66,14 @@ import { reactive, watch } from 'vue'
 import { useImportStore } from '../../stores/import.js'
 
 const store = useImportStore()
+// Downloader + schedule-defaults fields moved to
+// /subscriptions?tab=settings (operator decision 2026-05-27). This form
+// now only owns image-import filters.
 const local = reactive({
   min_width: 0, min_height: 0,
   skip_transparent: false, transparency_threshold: 0.9,
   skip_single_color: false, single_color_threshold: 0.95,
   phash_threshold: 10,
-  download_rate_limit_seconds: 3.0,
-  download_validate_files: true,
-  download_schedule_default_seconds: 28800,
-  download_event_retention_days: 90,
-  download_failure_warning_threshold: 5,
 })
 
 watch(() => store.settings, (s) => { if (s) Object.assign(local, s) }, { immediate: true })