perf(db): the channel manager and the feed sidebar read an index, not the video heap

GET /api/channels summed every subscribed channel's videos by reading each video's heap page
(681 MB on prod): 51-61 ms warm, but 2.25 s on a first open and 6.4 s under EXPLAIN once the
table had left memory. /feed/summary and /feed/years did the same (~140 ms warm, ~800 ms cold).
Migration 0095 replaces ix_videos_channel_pub_desc with the same key INCLUDING the columns
those queries read, so they are Index Only Scans over a 25 MB index. A test holds the queries
and the index's column list together.

The admin Configuration page did a db.get per setting (two for an unset one): it now reads
every override in one query (~40 ms of ~55 on prod).

backend/app/routes/channels.py

@@ -6,7 +6,7 @@ from datetime import datetime, timezone
 
 from fastapi import APIRouter, BackgroundTasks, Depends
 from pydantic import BaseModel
-from sqlalchemy import func, select, update
+from sqlalchemy import Select, func, select, update
 from sqlalchemy.orm import Session
 
 from app.collation import collated, sorted_words
@@ -40,6 +40,22 @@ router = APIRouter(prefix="/api/channels", tags=["channels"])
 log = logs.get_logger(__name__)
 
 
+def channel_aggregates(channel_ids: list[str]) -> Select:
+    """Per-channel stored count, last upload and total duration, in one grouped pass. Every column it
+    reads is in `ix_videos_channel_pub_cover` (migration 0095), so Postgres answers it from the index
+    alone; a test holds that (test_channel_list_index_only)."""
+    return (
+        select(
+            Video.channel_id,
+            func.count(Video.id),
+            func.max(Video.published_at),
+            func.coalesce(func.sum(Video.duration_seconds), 0),
+        )
+        .where(Video.channel_id.in_(channel_ids))
+        .group_by(Video.channel_id)
+    )
+
+
 @router.get("")
 def list_channels(user: User = Depends(current_user), db: Session = Depends(get_db)) -> list[dict]:
     rows = db.execute(
@@ -49,24 +65,15 @@ def list_channels(user: User = Depends(current_user), db: Session = Depends(get_
         .order_by(Subscription.priority.desc(), collated(Channel.title, lang_for_user(user)))
     ).all()
 
-    # Per-channel aggregates over the shared catalog in ONE grouped pass: stored count, last
-    # upload, total duration. Cheap — a single grouped scan, so computed on read rather than
-    # denormalised (measure-first: reads aren't frequent/hot enough to warrant cached columns).
+    # Per-channel aggregates over the shared catalog (channel_aggregates), computed on read rather than
+    # denormalised: an Index Only Scan over a ~20 MB index, where it used to read every video's heap page
+    # (681 MB on prod, a 2–6 s first open once something else had pushed it out of memory, E31 S4d).
     # (The Normal/Shorts/Live breakdown moved to the channel-detail page in R14, so it is no longer
     # computed here — the manager table dropped its N/S/L column.)
     channel_ids = [c.id for _, c in rows]
     agg: dict[str, dict] = {}
     if channel_ids:
-        for cid, n, last_up, total_dur in db.execute(
-            select(
-                Video.channel_id,
-                func.count(Video.id),
-                func.max(Video.published_at),
-                func.coalesce(func.sum(Video.duration_seconds), 0),
-            )
-            .where(Video.channel_id.in_(channel_ids))
-            .group_by(Video.channel_id)
-        ).all():
+        for cid, n, last_up, total_dur in db.execute(channel_aggregates(channel_ids)).all():
             agg[cid] = {
                 "stored": n,
                 "last_video_at": iso(last_up),