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.

scripts/e2e_seed.py

@@ -24,6 +24,7 @@ sys.path.insert(0, "/app")
 from sqlalchemy import delete, func, select
 
 from app.db import SessionLocal
+import app.tables  # noqa: F401 — every table, before the first query
 from app.catalog.models import Channel, Subscription, Video
 from app.models import AppState, User
 from app.reader.models import Playlist, PlaylistItem, VideoState