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

Channels, search and sync do no database work in their routes any more: reads go through
catalog/queries, writes through catalog/service, and a live search's ingest is catalog/search.
What a catalog route needs from Reader (a playlist's channels, a user's tag links, the feed
projection of results) it asks reader/queries and reader/service. The scheduler and the runs page
read channel titles and the work queue through catalog/queries. Catalog is fenced below its HTTP
layer: its owner filters go through userscope, it reads no other unit's tables, and its ingest
jobs commit only where they are named.

backend/app/scheduler.py

@@ -9,14 +9,13 @@ from typing import Callable
 from apscheduler.executors.pool import ThreadPoolExecutor
 from apscheduler.jobstores.base import ConflictingIdError
 from apscheduler.schedulers.background import BackgroundScheduler
-from sqlalchemy import select
 
 from app import audit, debugsession, jobruns, logs, logsink, progress, quota, sysconfig
 from app.config import settings
 from app.db import SessionLocal
 from app.downloads.gc import run_download_gc
 from app.models import JobRun
-from app.catalog.models import Channel
+from app.catalog import queries as catalog_queries
 from app.plex.sync import sync as run_plex_sync
 from app.plex.tmdb import run_enrich as run_tmdb_enrich
 from app.plex.tmdb import run_people_enrich
@@ -306,7 +305,7 @@ def scheduler_snapshot(db) -> dict:
             "running": going is not None,
             "target": shown.target if shown else None,
             "target_title": (
-                db.scalar(select(Channel.title).where(Channel.id == shown.target))
+                catalog_queries.channel_title(db, shown.target)
                 if shown is not None and shown.target
                 else None
             ),