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_unit_fences.py

@@ -110,13 +110,8 @@ UNIT_ROUTES = {
 # What each HTTP file still does, by file: the debt a unit's sprint pays off. Exact counts. A core
 # route's count is only the unit tables it imports; it is paid by the sprint of the unit it reaches.
 ALLOWED = {
-    "plex/_shared.py": 1,
     "plex/admin.py": 12,
-    "plex/browse.py": 93,
-    "plex/collections.py": 33,
-    "plex/detail.py": 29,
     "plex/discover.py": 44,
-    "plex/people.py": 20,
     "plex/playback.py": 42,
     "plex/playlists.py": 80,
 }