feat(provenance+tags): collapse provenance descriptions; purge retired-kind tags

Operator-flagged 2026-05-28, two asks.

**1. Provenance posts ate the panel.** Each post card rendered its full
description (180px scroll box) inline, so a few posts pushed everything
else off-screen. ProvenancePanel now collapses the description by
default behind a per-post "Show description ▾ / Hide ▴" toggle (state
keyed by provenance_id, reset when the viewed image changes). Cards stay
compact — platform/date/title/meta/actions — and the operator expands
only the descriptions they want.

**2. Purge tags of retired/system kinds.** The IR migration left
`archive`/`post`/`artist`-kind tags (e.g. `BlenderKnight:Hannah_BJ_Loops`)
that FC no longer creates — the tag input only makes
character/fandom/series/general, and provenance + artists are their own
systems now. (meta/rating were already hard-deleted by alembic 0023.)

- cleanup_service.purge_tags_by_kind(kinds=PURGEABLE_TAG_KINDS) — counts
  (dry_run) or deletes tags whose kind ∈ (archive, post, artist).
  CASCADE clears the image_tag / alias / allowlist / etc. rows.
- POST /api/admin/tags/purge-retired-kinds (Tier-A, dry-run preview
  returns per-kind counts + sample names — the preview IS the
  verification of exactly what'll be deleted before committing).
- Tag Maintenance card gets a second section: "Preview retired-kind
  tags" → per-kind breakdown + sample → "Delete N retired-kind tag(s)".

Tests: dry-run counts by kind (general survives), commit deletes only
the retired kinds (general + character survive, retired count → 0).

NOTE: a dry-run preview will show exactly which kinds/counts are
present. If the operator's noisy tags turn out to be `general` (e.g. an
IR `source:patreon` that fell back to general during migration), they
won't be caught by the kind purge — the preview makes that visible so
we can decide on a name-based pass separately.

backend/app/api/admin.py

@@ -206,3 +206,22 @@ async def tags_prune_unused():
             )
         )
     return jsonify(result)
+
+
+@admin_bp.route("/tags/purge-retired-kinds", methods=["POST"])
+async def tags_purge_retired_kinds():
+    """Tier-A: delete tags whose kind is a system/legacy kind FC no
+    longer creates (archive / post / artist — the IR-migration leftovers
+    like `BlenderKnight:Hannah_BJ_Loops`). dry-run preview returns
+    per-kind counts + a sample so the UI can show exactly what'll go
+    before the operator confirms with dry_run=false."""
+    from ..services.cleanup_service import purge_tags_by_kind
+
+    body = await request.get_json(silent=True) or {}
+    dry_run = bool(body.get("dry_run", False))
+
+    async with get_session() as session:
+        result = await session.run_sync(
+            lambda sync_sess: purge_tags_by_kind(sync_sess, dry_run=dry_run)
+        )
+    return jsonify(result)