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/explore.py

@@ -11,7 +11,7 @@ grace period (mirrors the maintenance job's grace-then-delete safety)."""
 import logging
 from datetime import timedelta
 
-from sqlalchemy import delete, select
+from sqlalchemy import delete, or_, select
 from sqlalchemy.orm import Session
 
 from app import sysconfig
@@ -31,6 +31,23 @@ from app.youtube.client import YouTubeClient
 log = logging.getLogger("siftlode.explore")
 
 
+def followed_channel():
+    """WHERE clause: a channel somebody follows — a subscription, or an exploration still inside its
+    grace window (`purge_unkept_explored` deletes the expired ExploredChannel rows first). ONE
+    definition for both halves of the question: what the scheduled RSS poll and recent backfill cover
+    (sync/runner.py) and what the explore purge may delete.
+
+    The poll and 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 _enrich_new(db: Session, yt: YouTubeClient, video_ids: list[str]) -> int:
     """Enrich just the not-yet-enriched videos among `video_ids` (videos.list, 1 unit/50)."""
     if not video_ids:
@@ -98,12 +115,8 @@ def purge_unkept_explored(db: Session) -> dict:
     ).rowcount
     db.commit()
 
-    sub_exists = select(Subscription.id).where(Subscription.channel_id == Channel.id).exists()
-    exp_exists = select(ExploredChannel.id).where(ExploredChannel.channel_id == Channel.id).exists()
     orphan_ids = (
-        db.execute(
-            select(Channel.id).where(Channel.from_explore.is_(True), ~sub_exists, ~exp_exists)
-        )
+        db.execute(select(Channel.id).where(Channel.from_explore.is_(True), ~followed_channel()))
         .scalars()
         .all()
     )