refactor(capture): the HTTP layer reads through queries and writes through the service

Capture's routes (downloads, files, public watch links) and the token list no longer build SQL, call
the session or set a model's fields: reads are downloads/queries, writes the unit's service (service,
folders, links, quota), and each write flushes for the handler to commit. Whose a row is goes
through userscope (owned_by, owned_row), which owned_or_404 now uses too.

The fence holds Capture below its routes as well: it reads only Catalog's and Plex's tables, writes
no other unit's, filters owners through userscope and commits only in its jobs and the upload
ingest. A test per service use case, among them that the tray lists only the caller's files.

backend/tests/test_download_limits.py

@@ -180,6 +180,7 @@ def test_a_suspended_users_queued_jobs_wait(db):
 
 
 def test_list_downloads_is_capped(db, monkeypatch):
+    from app.downloads import queries
     from app.routes import downloads as dl
 
     u = User(email="heavy@example.com")
@@ -189,7 +190,7 @@ def test_list_downloads_is_capped(db, monkeypatch):
         _job(db, u.id, queue_pos=i, status="done")
     db.commit()
 
-    monkeypatch.setattr(dl, "MAX_LIST", 2)
+    monkeypatch.setattr(queries, "MAX_LIST", 2)
     rows = dl.list_downloads(user=u, db=db)
     assert len(rows) == 2  # bounded by the cap, not the 3 rows on disk