refactor(catalog): the unit's tables move into catalog/models

Channel, Subscription, Video, SearchFind, ExploredChannel and BlockedChannel (and LIVE_OR_UPCOMING)
leave app/models.py for app/catalog/models.py; app.tables registers them. The User↔Subscription
relationship names its other side as a type only. No schema change.

backend/app/routes/channels.py

@@ -15,13 +15,13 @@ from app.mailtext import lang_for_user
 from app import logs, quota, scheduler
 from app.auth import admin_user, current_user, require_write_scope
 from app.db import get_db
-from app.models import (
+from app.models import User
+from app.catalog.models import (
     LIVE_OR_UPCOMING,
     BlockedChannel,
     Channel,
     ExploredChannel,
     Subscription,
-    User,
     Video,
 )
 from app.reader.models import ChannelTag, Playlist, PlaylistItem, Tag

backend/app/routes/runs.py

@@ -18,7 +18,8 @@ from sqlalchemy.orm import Session
 from app import scheduler
 from app.auth import admin_user
 from app.db import get_db
-from app.models import Channel, JobRun, LogEvent, User
+from app.models import JobRun, LogEvent, User
+from app.catalog.models import Channel
 from app.routes import _account as account
 from app.routes._keyset import PAGE_MAX, accounts, older_than, page, utc
 from app.runstatus import RunStatus

backend/app/routes/scheduler.py

@@ -12,7 +12,8 @@ from app.audit import AuditAction
 from app.auth import admin_user
 from app.config import settings
 from app.db import get_db
-from app.models import Channel, Subscription, User, Video
+from app.models import User
+from app.catalog.models import Channel, Subscription, Video
 from app.scheduler import (
     JOB_INTERVALS,
     MAX_INTERVAL,

backend/app/routes/search.py

@@ -25,13 +25,13 @@ from sqlalchemy.orm import Session, aliased
 from app import logs, quota, sysconfig
 from app.auth import current_user
 from app.db import get_db
-from app.models import (
+from app.models import User
+from app.catalog.models import (
     LIVE_OR_UPCOMING,
     BlockedChannel,
     Channel,
     SearchFind,
     Subscription,
-    User,
     Video,
 )
 from app.reader.models import Playlist, PlaylistItem, VideoState

backend/app/routes/sync.py

@@ -6,7 +6,8 @@ from app import audit, quota, state
 from app.audit import AuditAction
 from app.auth import admin_user, current_user, has_read_scope
 from app.db import get_db
-from app.models import Channel, Subscription, User, Video
+from app.models import User
+from app.catalog.models import Channel, Subscription, Video
 from app.routes._common import youtube_refusal
 from app.ratelimit import RateLimiter
 from app.sync.runner import estimate_deep_backfill