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.

tests/test_api_ml_admin.py

@@ -19,7 +19,12 @@ async def test_get_and_patch_settings(client):
     resp = await client.get("/api/ml/settings")
     assert resp.status_code == 200
     body = await resp.get_json()
-    assert body["suggestion_threshold_general"] == pytest.approx(0.95)
+    # Default lowered 0.95 → 0.50 on 2026-06-01 (alembic 0029) — 0.95
+    # hid most general suggestions in the view modal.
+    assert body["suggestion_threshold_general"] == pytest.approx(0.50)
+    # Retired threshold columns must not appear in the payload.
+    assert "suggestion_threshold_artist" not in body
+    assert "suggestion_threshold_copyright" not in body
 
     resp = await client.patch(
         "/api/ml/settings", json={"suggestion_threshold_general": 0.90}