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/sysconfig.py

@@ -10,6 +10,7 @@ stored in the DB and stay env-only).
 
 from dataclasses import dataclass
 
+from sqlalchemy import select
 from sqlalchemy.orm import Session
 
 from app import logs, security
@@ -307,7 +308,11 @@ def _raw_override(db: Session, key: str) -> str | None:
     per key so the operator gets a real signal without a per-request traceback. Recovery is documented in
     docs/self-hosting.md: the key must never change once secrets are stored. (B-core-2)
     """
-    row = db.get(SystemConfig, key)
+    return _raw_of(db.get(SystemConfig, key), key)
+
+
+def _raw_of(row: SystemConfig | None, key: str) -> str | None:
+    """`_raw_override` over a row already read (describe() reads them all in one query)."""
     if row is None or row.value is None:
         return None
     if not row.encrypted:
@@ -327,8 +332,11 @@ def _raw_override(db: Session, key: str) -> str | None:
 
 def get(db: Session, key: str):
     """Effective value: the DB override (typed) if set, else the env/config default."""
-    s = _BY_KEY[key]
-    raw = _raw_override(db, key)
+    return _effective(_BY_KEY[key], _raw_override(db, key))
+
+
+def _effective(s: ConfigSpec, raw: str | None):
+    """`get` over an override already read."""
     if raw is None:
         return _default(s)
     if raw == "" and not s.allow_empty:
@@ -420,8 +428,12 @@ def describe(db: Session) -> dict:
     """Admin-UI metadata + current values, grouped. Secret values are never echoed —
     only whether an override is set and whether an env default exists."""
     groups: dict[str, list[dict]] = {}
+    # Every override in one query: a db.get per key was one round trip each (two for a key with no
+    # row, which the session cannot cache), ~40 ms of the page's ~55 on prod (E31 S4d).
+    rows = {r.key: r for r in db.scalars(select(SystemConfig))}
     for s in SPECS:
-        is_set = db.get(SystemConfig, s.key) is not None
+        row = rows.get(s.key)
+        is_set = row is not None
         item: dict = {
             "key": s.key,
             "type": s.type,
@@ -436,7 +448,7 @@ def describe(db: Session) -> dict:
         if s.secret:
             item["default_is_set"] = bool(_default(s))
         else:
-            item["value"] = get(db, s.key)
+            item["value"] = _effective(s, _raw_of(row, s.key))
             item["default"] = _default(s)
         groups.setdefault(s.group, []).append(item)
     sections = [