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/auth.py

@@ -972,17 +972,6 @@ async def upgrade(
     )
 
 
-@router.get("/me")
-async def me(user: User = Depends(current_user)) -> dict:
-    return {
-        "id": user.id,
-        "email": user.email,
-        "display_name": user.display_name,
-        "avatar_url": user.avatar_url,
-        "role": user.role,
-    }
-
-
 @router.get("/config")
 def auth_config(db: Session = Depends(get_db)) -> dict:
     """Public: which sign-in options this instance offers, so the login page can hide what's off