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_route_conventions.py

@@ -40,7 +40,7 @@ FUNCTIONS = (ast.FunctionDef, ast.AsyncFunctionDef)
 # of them — `{"synced": n}` is an acknowledgement, and gets spread into `{"ok": True, **report}`.
 SERIALIZERS = {
     "JSONResponse",
-    "_collection_card",
+    "collection_card",
     "_playlist_card",
     "_serialize",
     "_serialize_folder",