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/routes/downloads.py

@@ -265,7 +265,7 @@ def enqueue_download(
             db, user.id, source_kind, source_ref, spec, profile_id, display_name
         )
     except quota.QuotaExceeded as e:
-        raise HTTPException(status_code=422, detail=_quota_message(e))
+        raise HTTPException(status_code=422, detail=_quota_detail(e))
     return _serialize_job(db, job)
 
 
@@ -288,27 +288,16 @@ def enqueue_edit(
     try:
         job = service.enqueue_edit(db, user.id, source_job, spec, display_name)
     except quota.QuotaExceeded as e:
-        raise HTTPException(status_code=422, detail=_quota_message(e))
+        raise HTTPException(status_code=422, detail=_quota_detail(e))
     except service.EditError as e:
-        raise HTTPException(status_code=400, detail=_edit_message(e))
+        raise HTTPException(status_code=400, detail={"code": "edit", "reason": e.reason})
     return _serialize_job(db, job)
 
 
-def _edit_message(e: service.EditError) -> str:
-    if e.reason == "empty_edit":
-        return "Choose a trim range or crop area first."
-    if e.reason == "source_not_ready":
-        return "The source download isn't ready to edit."
-    return "That edit couldn't be created."
-
-
-def _quota_message(e: quota.QuotaExceeded) -> str:
-    if e.reason == "max_jobs":
-        return f"You've reached your download limit ({e.limit} items). Remove some first."
-    if e.reason == "max_bytes":
-        gb = e.limit / 1_073_741_824
-        return f"You've used your download storage quota ({gb:.1f} GB). Free up space first."
-    return "Download quota exceeded."
+def _quota_detail(e: quota.QuotaExceeded) -> dict:
+    """Structured error the trilingual client localizes (reason key + numbers), instead of a
+    pre-baked English string with a dot-decimal GB value. See lib/api.ts localizeDetail()."""
+    return {"code": "quota", "reason": e.reason, "limit": e.limit, "current": e.current}
 
 
 @router.get("")