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.

backend/app/api/downloads.py

@@ -5,8 +5,10 @@ status/source/artist. Returns slim records.
 Detail view: full DownloadEvent including the metadata JSONB.
 """
 
+from datetime import datetime, timedelta, timezone
+
 from quart import Blueprint, jsonify, request
-from sqlalchemy import select
+from sqlalchemy import func, select
 
 from ..extensions import get_session
 from ..models import Artist, DownloadEvent, Source
@@ -95,6 +97,35 @@ async def list_downloads():
     return jsonify([_list_record(e, s, a) for e, s, a in rows])
 
 
+@downloads_bp.route("/stats", methods=["GET"])
+async def downloads_stats():
+    """Status-grouped count over download_event for the dashboard stat chips.
+
+    `?window_hours=` (default 24) bounds by `started_at`. The full set of
+    statuses is always present in the response (zero for missing) so the
+    UI doesn't have to fill in defaults.
+    """
+    try:
+        window_hours = int(request.args.get("window_hours", "24"))
+    except ValueError:
+        return jsonify({"error": "invalid_window_hours"}), 400
+    if window_hours < 1 or window_hours > 24 * 365:
+        return jsonify({"error": "invalid_window_hours"}), 400
+
+    since = datetime.now(timezone.utc) - timedelta(hours=window_hours)
+    out = {"pending": 0, "running": 0, "ok": 0, "error": 0, "skipped": 0}
+    async with get_session() as session:
+        stmt = (
+            select(DownloadEvent.status, func.count())
+            .where(DownloadEvent.started_at >= since)
+            .group_by(DownloadEvent.status)
+        )
+        for status, n in (await session.execute(stmt)).all():
+            if status in out:
+                out[status] = int(n)
+    return jsonify(out)
+
+
 @downloads_bp.route("/<int:event_id>", methods=["GET"])
 async def get_download(event_id: int):
     async with get_session() as session: