refactor(plex): the browse, detail, people and collection pages read through queries and write through the service

The HTTP files browse, detail, people, collections and _shared do no database work of their own
now. New plex/queries.py (the rows a page shows, a user's watch state through userscope),
plex/service.py (the info pages' enrichment, a collection's editable flag, the row kept after a
failed follow-up read, the person bio's background fetch; none commits), plex/library.py (the
unified browse feed, its search and facets) and plex/collections.py (a collection's source,
editability, card, certificate and member order). Every watch-state join in the moved code names
its owner through owned_by.

Pins 354 -> 178 in 4 files; basedpyright baseline 132 -> 118.

backend/tests/test_plex_people_enrich.py

@@ -362,7 +362,7 @@ def test_the_person_page_says_a_bio_is_on_its_way(db, monkeypatch, override_dep)
     monkeypatch.setattr(state, "_configured_cache", True)
     override_dep(get_db, db.info["request"])
     override_dep(current_user, lambda: user)
-    monkeypatch.setattr(people, "_enrich_person_bg", lambda pid: None)
+    monkeypatch.setattr(people.service, "enrich_person_bg", lambda pid: None)
     key = {"value": "key"}
     monkeypatch.setattr(people.sysconfig, "get_str", lambda d, k: key["value"])