fix(capture): every process registers every table before its first query

The models now live in their units' packages, and a core model names a unit's by
string (User.subscriptions -> Subscription). The download child imports neither
catalog nor app.tables before db.get, so mapper configuration failed and every
download, clip, edit and upload job died with child_exit. Each entry point now
imports app.tables, and a test imports each one in a clean interpreter.

backend/app/worker.py

@@ -31,6 +31,7 @@ from functools import partial
 from sqlalchemy import text
 from sqlalchemy.orm import Session
 
+import app.tables  # noqa: F401 — every table, before the first query
 from app import logs
 from app import logsink
 from app import sysconfig