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/modal/AliasPickerDialog.vue

@@ -48,10 +48,11 @@ function onSearch(q) {
     if (!query) { results.value = []; return }
     loading.value = true
     try {
-      // Scope the autocomplete to the prediction's category where it maps
-      // to a tag kind. 'copyright' has no tag kind; search unscoped there.
-      const kind = ['artist', 'character'].includes(props.category)
-        ? props.category : null
+      // Scope the autocomplete to the prediction's category where it
+      // maps to a tag kind. Only 'character' surfaces as both a
+      // suggestion category and a tag kind now ('artist' + 'copyright'
+      // retired); other categories search unscoped.
+      const kind = props.category === 'character' ? 'character' : null
       const params = { q: query, limit: 20 }
       if (kind) params.kind = kind
       results.value = await api.get('/api/tags/autocomplete', { params })