refactor: one definition of a followed channel and of a choice spelling

The explore purge rebuilt followed_channel's two EXISTS clauses inline, so what is polled and what may be deleted could drift; it now uses the predicate, which moved to sync/explore.py. The env validator and sysconfig each folded a choice value on their own; both call config.canonical_choice. The deep-backfill comment no longer calls recent backfill unconditional.

backend/app/sync/runner.py

@@ -10,14 +10,15 @@ import threading
 from collections import Counter
 from concurrent.futures import ThreadPoolExecutor, as_completed
 
-from sqlalchemy import or_, select
+from sqlalchemy import select
 from sqlalchemy.orm import Session
 
 from app import logs, progress, quota, sysconfig
 
 log = logging.getLogger("siftlode.sync")
 from app.db import SessionLocal
-from app.models import Channel, ExploredChannel, OAuthToken, Subscription, User
+from app.models import Channel, OAuthToken, Subscription, User
+from app.sync.explore import followed_channel
 from app.sync.subscriptions import import_subscriptions
 from app.sync.videos import (
     apply_rss_feed,
@@ -54,20 +55,6 @@ def get_service_user(db: Session) -> User | None:
     return users[0] if users else None
 
 
-def followed_channel():
-    """WHERE clause: a channel somebody follows — a subscription, or an exploration still inside its
-    grace window. The scheduled RSS poll and recent backfill cover only these (owner decision,
-    2026-09-17). They used to cover EVERY channel row, but rows also come from search results,
-    playlist sync and dropped subscriptions, and nothing deletes those: on prod 582 of 979 polled
-    channels had no follower, three of them dead feeds answering 404 on every poll. The cost of
-    narrowing is deliberate: the shared Library stops receiving new uploads from channels nobody
-    follows."""
-    return or_(
-        select(Subscription.id).where(Subscription.channel_id == Channel.id).exists(),
-        select(ExploredChannel.id).where(ExploredChannel.channel_id == Channel.id).exists(),
-    )
-
-
 def run_rss_poll(db: Session, channels: list[Channel] | None = None) -> dict[str, int]:
     """Poll the free RSS feed of every followed channel (`followed_channel`). Returns `{"new": inserted, "failed": channels_that_did
     _not_complete}` — the failure count is part of the RESULT, not just a log line, so the scheduler
@@ -258,7 +245,8 @@ def run_deep_backfill(db: Session, max_channels: int = 10, max_pages: int = 5) -
     if user is None:
         return {"channels_processed": 0, "videos_added": 0, "reason": "no service user"}
     # Demand-driven: only deep-backfill channels at least one user has opted into
-    # (Subscription.deep_requested). Recent backfill stays unconditional (cheap).
+    # (Subscription.deep_requested). Recent backfill needs no opt-in: it covers every followed channel
+    # (`followed_channel`).
     deep_requested = (
         select(Subscription.id)
         .where(