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/tests/test_sysconfig_describe_queries.py

@@ -0,0 +1,39 @@
+"""The admin Configuration page reads every override in one query (E31 S4d).
+
+`sysconfig.describe()` did a `db.get` per key to tell whether an override was set, and `get()` did one
+more for its value: ~55 keys, and two round trips for a key with no row, since the session cannot cache
+an absence. That was ~40 ms of the page's ~55 on prod. It now reads the table once. DB lane.
+"""
+
+from sqlalchemy import event
+
+from app import sysconfig
+
+
+def _statements(db, fn) -> int:
+    seen: list[str] = []
+    engine = db.get_bind()
+    listener = lambda *a, **k: seen.append(a[2])  # noqa: E731 — (conn, cursor, statement, …)
+    event.listen(engine, "before_cursor_execute", listener)
+    try:
+        fn()
+    finally:
+        event.remove(engine, "before_cursor_execute", listener)
+    return len(seen)
+
+
+def test_describe_reads_the_overrides_once(db):
+    sysconfig.set_value(db, "quota_daily_budget", 5000)
+    db.commit()
+    assert _statements(db, lambda: sysconfig.describe(db)) == 1
+
+
+def test_describe_shows_the_stored_value_and_the_default(db):
+    sysconfig.set_value(db, "quota_daily_budget", 5000)
+    db.commit()
+    items = {i["key"]: i for g in sysconfig.describe(db)["groups"].values() for i in g}
+    assert items["quota_daily_budget"]["is_set"] is True
+    assert items["quota_daily_budget"]["value"] == 5000
+    other = next(i for k, i in items.items() if k != "quota_daily_budget" and not i["secret"])
+    assert other["is_set"] is False
+    assert other["value"] == other["default"]