feat(modal): autofocus tag input, expand general suggestions, retire copyright/artist categories

Four coupled operator-asked changes to the view modal (Scribe plan #509):

1. **Autofocus tag entry on modal open** — TagAutocomplete grabs focus
   in onMounted/nextTick so the caret is in the input the moment the
   modal renders. No click needed to start typing.

2. **General suggestions expanded by default** — SuggestionsPanel's
   general-category group now mounts with `:default-open="true"`.
   Operator can collapse if too noisy, but the v1 frame shows them.

3. **Lower general threshold default 0.95 → 0.50** — MLSettings.
   suggestion_threshold_general default matches character. Alembic
   0029 also bumps the existing singleton row's value if it's still
   at the old 0.95. Operator can re-tune from Settings → ML.

4. **Retire `copyright` + `artist` as ML suggestion categories** —
   neither feeds a Tag.kind (`artist` retired in FC-2d-vii-c, never
   really existed as a copyright tag-kind). They were surfaced in the
   suggestions pipeline + threshold settings UI but had no follow-
   through. Drop from SURFACED_CATEGORIES, suggestions._threshold_for,
   ml_admin GET/PATCH allowlist, MLSettings columns (alembic 0029
   drops the two columns), frontend CATEGORY_ORDER + CATEGORY_LABELS,
   SuggestionsPanel.peopleCats, AliasPickerDialog kind-check, and
   MLThresholdSliders rows.

Out of scope (intentional): `tag_kind` Postgres enum still includes
`artist` for historic Tag row queryability (per the model comment);
no operator pain reported, no enum-shrink needed.

Tests:
- test_surfaced_categories asserts {character, general}, excludes
  artist + copyright.
- test_threshold_for_artist_is_unsurfaced extended to cover copyright.
- test_get_and_patch_settings asserts new 0.50 default and the absent
  artist + copyright keys in the GET payload.

frontend/src/components/settings/MLThresholdSliders.vue

@@ -22,10 +22,10 @@ import { reactive, watch } from 'vue'
 import { useMLStore } from '../../stores/ml.js'
 
 const store = useMLStore()
+// 'artist' (FC-2d-vii-c) and 'copyright' (2026-06-01) retired as
+// suggestion categories; their threshold rows are gone.
 const fields = [
-  { key: 'suggestion_threshold_artist', label: 'Artist' },
   { key: 'suggestion_threshold_character', label: 'Character' },
-  { key: 'suggestion_threshold_copyright', label: 'Copyright' },
   { key: 'suggestion_threshold_general', label: 'General' },
   { key: 'centroid_similarity_threshold', label: 'Centroid similarity' }
 ]