test(db): /facets regression + S3a jsonb-null normalization (R6 S3)

First real customers of the DB lane:
- test_plex_facets: GET /api/plex/facets?scope=show returns 200 (not the
  "cannot extract elements from a scalar" 500) for a show whose genres is a JSONB
  scalar `null` — the 0.53.1 read-guard's regression test. Mutation-checked:
  removing the jsonb_typeof='array' guard turns it red with that exact error.
- migration 0060 (S3a): one-time UPDATE …=NULL WHERE jsonb_typeof=… 'null' over the
  plex_items/plex_shows tag columns, so pre-0.53.1 JSONB-null rows become uniform SQL
  NULL and the read-guards go belt-only. test_plex_jsonb_normalize verifies scalar-null
  → SQL NULL while a real array is preserved.

Gate: ruff clean; DB lane 176 passed; pure-logic lane unchanged (skips the DB tests).

backend/tests/test_plex_facets.py

@@ -0,0 +1,45 @@
+"""DB-backed regression for the 0.53.1 hotfix (R6 S3b-test): GET /api/plex/facets?scope=show
+must return 200 — not the "cannot extract elements from a scalar" 500 — for a metadata-poor
+library whose show has a JSONB-scalar `null` genres (a pre-0.53.1 row). The endpoint's
+`jsonb_typeof(genres)='array'` read-guard is what makes it safe; this test would 500 without it.
+
+Uses the DB lane (`db` fixture) — skipped when no Postgres is reachable.
+"""
+from fastapi.testclient import TestClient
+from sqlalchemy import text
+
+import app.state as state
+from app.auth import current_user
+from app.db import get_db
+from app.main import app
+from app.models import PlexLibrary, PlexShow, User
+
+
+def test_facets_survives_json_null_genres(db):
+    lib = PlexLibrary(plex_key="1", title="Shows", kind="show", enabled=True)
+    db.add(lib)
+    db.flush()
+    db.add(PlexShow(rating_key="s1", library_id=lib.id, title="No Genre Show"))
+    user = User(email="facets@example.com")
+    db.add(user)
+    # The request gate 503s an unconfigured instance; mark this throwaway DB configured.
+    state.mark_configured(db)
+    db.commit()
+    # Simulate a pre-0.53.1 row: genres persisted as a JSONB scalar `null` (what SQLAlchemy wrote
+    # before none_as_null=True), NOT SQL NULL — this is the value jsonb_array_elements_text() chokes
+    # on. A Python None now maps to SQL NULL, so we set the scalar directly.
+    db.execute(text("UPDATE plex_shows SET genres = 'null'::jsonb WHERE rating_key = 's1'"))
+    db.commit()
+
+    app.dependency_overrides[get_db] = lambda: db
+    app.dependency_overrides[current_user] = lambda: user
+    try:
+        client = TestClient(app)  # no context-manager: skip lifespan (scheduler/HLS sweep)
+        resp = client.get("/api/plex/facets?scope=show")
+    finally:
+        app.dependency_overrides.clear()
+        state._configured_cache = False  # one-way process cache — don't leak "configured" to other tests
+
+    assert resp.status_code == 200, resp.text
+    # The scalar-null show contributes no genres — and, crucially, doesn't abort the whole endpoint.
+    assert resp.json()["genres"] == []