refactor(routes): delete 5 dead endpoints the frontend never calls (E3 S3, B-core-8)

None are referenced by the SPA or tests; the Scheduler dashboard's "run now"
covers every manual trigger. Removed: POST /api/sync/{rss,backfill,enrich}
(+ the now-dead _user_channels helper and three sync.runner imports),
POST /api/tags/recompute (+ its admin_user / run_autotag_all imports), and
GET /auth/me (a lesser duplicate of the canonical /api/me in routes/me.py).

This also retires a latent bug: POST /api/sync/backfill reset backfill_done /
backfill_cursor on already-fully-backfilled channels (tipping them back to
"needs full history") for up to 25 channels per curl. Full lane 365 pass.

backend/app/routes/sync.py

@@ -7,30 +7,13 @@ from app.audit import AuditAction
 from app.auth import admin_user, current_user, has_read_scope, require_human
 from app.db import get_db
 from app.models import Channel, Subscription, User, Video
-from app.sync.runner import (
-    estimate_deep_backfill,
-    run_enrich,
-    run_recent_backfill,
-    run_rss_poll,
-)
+from app.sync.runner import estimate_deep_backfill
 from app.sync.subscriptions import import_subscriptions
 from app.scheduler import running_job_ids
 
 router = APIRouter(prefix="/api/sync", tags=["sync"])
 
 
-def _user_channels(db: Session, user: User) -> list[Channel]:
-    return (
-        db.execute(
-            select(Channel)
-            .join(Subscription, Subscription.channel_id == Channel.id)
-            .where(Subscription.user_id == user.id)
-        )
-        .scalars()
-        .all()
-    )
-
-
 @router.post("/subscriptions")
 def sync_subscriptions(
     user: User = Depends(require_human), db: Session = Depends(get_db)
@@ -48,38 +31,6 @@ def sync_subscriptions(
     return result
 
 
-@router.post("/rss")
-def sync_rss(
-    user: User = Depends(require_human), db: Session = Depends(get_db)
-) -> dict:
-    # A partial failure comes back in `failed_feeds` so a manual poll reports it instead of an
-    # innocent-looking "0 new"; a TOTAL outage raises out of run_rss_poll and surfaces as a 500,
-    # which is the honest answer for a poll that reached nothing.
-    result = run_rss_poll(db, _user_channels(db, user))
-    return {"new_videos": result["new"], "failed_feeds": result["failed"]}
-
-
-@router.post("/backfill")
-def sync_backfill(
-    user: User = Depends(require_human),
-    db: Session = Depends(get_db),
-    max_channels: int = 25,
-) -> dict:
-    with quota.measured(db, user.id, quota.QuotaAction.VIDEOS_BACKFILL_RECENT) as m:
-        result = run_recent_backfill(db, _user_channels(db, user), max_channels=max_channels)
-    result["quota_used_estimate"] = m.used
-    return result
-
-
-@router.post("/enrich")
-def sync_enrich(
-    user: User = Depends(require_human), db: Session = Depends(get_db)
-) -> dict:
-    with quota.measured(db, user.id, quota.QuotaAction.VIDEOS_ENRICH) as m:
-        enriched = run_enrich(db)
-    return {"enriched": enriched, "quota_used_estimate": m.used}
-
-
 @router.get("/status")
 def sync_status(
     user: User = Depends(current_user), db: Session = Depends(get_db)