fix(deferred): close backend correctness/efficiency tails from the hygiene sweep

Verified each deferred per-subsystem item against current source, then fixed the
real ones (tradeoffs left as-is, see siftlode-ops/CODE-HYGIENE.md closeout):

- search BUG-3: don't finalise shorts_probed on un-enriched stubs (enrichment
  failure/omitted rows) — restores the scheduler's enriched_at guard so a real
  Short can't leak into the feed and never get reclassified.
- downloads GC: 4th pass reaps orphaned errored, fileless, unreferenced assets
  (they carry no TTL, so the expiry passes never cleared them).
- downloads B6: quota/edit errors now return a structured {code,reason,limit}
  the trilingual client localises, instead of hardcoded English + dot-decimal GB.
- channels BB1: reset-backfill re-arms deep sync on every subscriber (bulk update)
  instead of 404ing when the admin isn't personally subscribed.
- channels BB2: enrich the stub on BOTH the normal and "already exists" desync
  path (nest the insert try inside the client `with`).
- channels CB3: drop the redundant second _discovery_rows read (identity-mapped
  rows are already enriched; re-sort in Python for the title tie-break).
- playlists PC1: read the live playlist once in push() and share it with plan_push
  + push_playlist (halves read-quota, closes the second TOCTOU window).
- playlists PC2/PC5: batch the cover thumbnails in one window query (was N+1);
  rename combines count+duration into one aggregate + a single cover lookup.
- messages MB2: get_thread only resolves a messageable partner OR one we already
  share a thread with (closes the user-enumeration oracle).
- messages MB3: push a live unread-count to the user's other tabs after mark-read.
- messages MC4: list_conversations uses DISTINCT ON + grouped COUNT instead of
  loading the whole message history into memory.
- config AB3: per-spec allow_empty so smtp_user/youtube_api_proxy can be blanked
  to disable them rather than snapping back to the env default.

backend/app/sync/playlists.py

@@ -217,11 +217,12 @@ def _reorder_moves(current: list[str], desired: list[str]) -> int:
     return moves
 
 
-def plan_push(db: Session, user: User, pl: Playlist) -> dict:
+def plan_push(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
     """Compute (without writing) what pushing this local playlist to YouTube would do:
     create vs update, how many inserts/deletes/reorders, the quota estimate, and whether
     YouTube has diverged (items present there that local will remove). Costs read units
-    (1/page) for a linked playlist; nothing for a fresh export."""
+    (1/page) for a linked playlist; nothing for a fresh export. `live` = a pre-fetched
+    (current_items, snippet) tuple so the caller can share one read with push_playlist."""
     desired = _local_video_ids(db, pl)
     if not pl.yt_playlist_id:
         ops = 1 + len(desired)  # create the playlist + one insert per video
@@ -234,9 +235,12 @@ def plan_push(db: Session, user: User, pl: Playlist) -> dict:
             "yt_extra": 0,
             "units_estimate": ops * WRITE_UNIT_COST,
         }
-    with YouTubeClient(db, user) as yt:
-        current = list(yt.iter_playlist_items_with_ids(pl.yt_playlist_id))
-        snippet = yt.get_playlist_snippet(pl.yt_playlist_id)
+    if live is not None:
+        current, snippet = live
+    else:
+        with YouTubeClient(db, user) as yt:
+            current = list(yt.iter_playlist_items_with_ids(pl.yt_playlist_id))
+            snippet = yt.get_playlist_snippet(pl.yt_playlist_id)
     rename = bool(snippet) and (snippet.get("title") or "") != pl.name
     cur_vids = [it["video_id"] for it in current]
     cur_set = set(cur_vids)
@@ -259,10 +263,11 @@ def plan_push(db: Session, user: User, pl: Playlist) -> dict:
     }
 
 
-def push_playlist(db: Session, user: User, pl: Playlist) -> dict:
+def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
     """Push a local playlist to YouTube: create it if needed, then reconcile membership
     and order so YouTube matches local (local wins). Marks the playlist clean on success.
-    Caller must have checked the write scope and the quota budget."""
+    Caller must have checked the write scope and the quota budget. `live` = a pre-fetched
+    (current_items, snippet) tuple (from plan_push's read) to avoid a second live page-through."""
     desired = _local_video_ids(db, pl)
     inserted = deleted = reordered = created = 0
     failures: list[str] = []
@@ -291,14 +296,17 @@ def push_playlist(db: Session, user: User, pl: Playlist) -> dict:
                 "yt_playlist_id": pl.yt_playlist_id,
             }
 
-        # Existing link: diff against the live YouTube state.
-        snippet = yt.get_playlist_snippet(pl.yt_playlist_id)
+        # Existing link: diff against the live YouTube state (reuse plan_push's read if given).
+        if live is not None:
+            current, snippet = live
+        else:
+            snippet = yt.get_playlist_snippet(pl.yt_playlist_id)
+            current = list(yt.iter_playlist_items_with_ids(pl.yt_playlist_id))
         if snippet is not None and (snippet.get("title") or "") != pl.name:
             try:
                 yt.update_playlist_title(pl.yt_playlist_id, pl.name)
             except YouTubeError:
                 failures.append("title")
-        current = list(yt.iter_playlist_items_with_ids(pl.yt_playlist_id))
         item_id_by_vid = {it["video_id"]: it["item_id"] for it in current}
         cur_vids = [it["video_id"] for it in current]
         desired_set = set(desired)