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/main.py

@@ -28,6 +28,7 @@ log = logs.get_logger(__name__)
 from app import bodylimit, collation, logsink, state, sysconfig  # noqa: E402 — logging is configured first
 from app.config import settings  # noqa: E402 — logging is configured first
 from app.db import SessionLocal, engine  # noqa: E402 — logging is configured first
+import app.tables  # noqa: E402 — every table, before the first query
 from app.downloads import service as download_service  # noqa: E402 — logging is configured first
 from app.downloads import storage  # noqa: E402 — logging is configured first
 from app.plex import stream as plex_stream  # noqa: E402 — logging is configured first