feat(ia): wave 1 — Import tab dissolves, Maintenance regroups by system, one extension home

Settings IA per the approved A3 design (the old layout was the two-app merge
fossilized):
- Import tab retired: ImportTriggerPanel + ImportTaskList deleted (manual
  /import scans stay API-level; imports arrive via downloads/extension, heal
  via the Layer-2 auto-refetch sweep, and show in Activity). ImportFiltersForm
  moves to Maintenance → 'Ingestion & filters' and loads its own settings; the
  import store shrinks to settings-only (no remaining consumers of the
  scan/task-list machinery). Overview's pending banner now points at Activity.
- Maintenance regrouped: Ingestion & filters / GPU agent & embeddings
  (GpuAgent, Failed processing, CPU embedding backfill) / Tagging (sliders,
  Heads, Aliases) / Library health (MissingFiles, Thumbnails, DB, Archive
  re-extract demoted last) / Storage.
- One extension home: BrowserExtensionCard moves from Settings → Overview to
  Subscriptions → Settings, above the API key bar it authenticates.
- Single-color import filter WIRED: skip_single_color/threshold existed since
  FC-2 but nothing read them (the audit module's docstring said as much) —
  now enforced on both import paths via the audit's canonical predicate
  (tolerance 30, matching the Cleanup card default; animated images exempt
  like the transparency check). Default stays off; test added.
- Dead weight: PlaceholderView (zero refs) and the permanently-disabled
  'Export failed logs (CSV — v2)' menu stub deleted; stale docs fixed
  (celery queue docstring, threshold comment citing retired tasks, ml
  package docstring, HeadsCard 'replaces Camie' blurb).

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

backend/app/tasks/maintenance.py

@@ -139,10 +139,9 @@ QUEUE_STUCK_THRESHOLD_MINUTES: dict[str, int] = {
     "download": 30,
     # Audit 2026-06-02 — maintenance/scan queues run tasks that
     # legitimately exceed the 5-min default (verify_integrity at 70m
-    # hard, scan_directory at 70m hard, apply_allowlist_tags /
-    # recompute_centroids / backfill_phash at 35m hard). 75 min lives
-    # above the longest of those and the per-task overrides below
-    # cover the outliers (backups, library audit).
+    # hard, scan_directory at 70m hard, backfill_phash at 35m hard).
+    # 75 min lives above the longest of those and the per-task
+    # overrides below cover the outliers (backups, library audit).
     "maintenance": 75,
     "scan": 75,
 }