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/app/routes/_common.py

@@ -16,6 +16,7 @@ from app.db import Base
 from app.models import POSITION_MAX_S, User
 from app.issues import gitea
 from app.refusal import refused
+from app.userscope import owned_row
 from app.youtube.client import YouTubeError, refusal_reason
 
 M = TypeVar("M", bound=Base)
@@ -90,8 +91,8 @@ def owned_or_404(
     else 404s (NOT 403) on purpose — a 403 would confirm the id exists to a stranger.
     It replaced the near-identical per-route ownership helpers, and is called directly — a thin
     `_own_x` wrapper around it is the ad-hoc shape CLAUDE.md's patterns table rules out."""
-    obj = get_or_404(db, model, id_, what=what, lock=lock)
-    if getattr(obj, owner_attr) != user.id:
+    obj = owned_row(db, model, id_, user, owner_attr=owner_attr, lock=lock)
+    if obj is None:
         raise not_found(what)
     return obj