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/services/audits/single_color.py

@@ -1,8 +1,9 @@
 """Single-color audit: matches images where one color dominates beyond
-the threshold (within the given Euclidean RGB tolerance). The first
-canonical implementation — the import-side filter (SkipReason.single_color)
-was never wired; FC-Cleanup's audit module is the source of truth and a
-future spec can adopt it on the import path too.
+the threshold (within the given Euclidean RGB tolerance). The canonical
+predicate for BOTH surfaces: FC-Cleanup's retroactive audit and — since
+2026-07-02 — the import-side filter (Importer._single_color_hit /
+SkipReason.single_color), so what the audit flags and what the import
+skips can never disagree.
 """
 
 from PIL import Image