bookstack: 26.05.5 -> 26.09.1 (within the line), both venues proven (R-462, Part F)

The ONLY image move in this commit. Written by upgrade-test.py --write-ladder:
- bench LXC 9401 (harness v4, the bench's own BookStack fixture): FROM settled, the seeded admin
  read back, migrations ran, read back after, 10-minute memory watch peak 43.7 % (anon), 0 kills,
  0 restarts; the abort (old image back) starts and serves the data;
- box 9202 (controller 0.283.1, the product's guarded Update, drill catalog): done in 64.6 s, the
  seeded admin read back through the app's own CLI.
The superseded step keeps its definition at steps/fdb7afbec5ea4dbc.{yml,felhom.yml} (09 §6.4 part 5).
The database half only, as before (R-460). MariaDB 12.3 does not move.
Evidence: felhom.eu/documentation/audits/pg-last-six-2026-09-30/F/ and .../box/bookstack/

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

templates/bookstack/docker-compose.yml

@@ -10,7 +10,7 @@
 
 services:
   bookstack:
-    image: lscr.io/linuxserver/bookstack:26.05.5
+    image: lscr.io/linuxserver/bookstack:26.09.1
     container_name: bookstack
     restart: unless-stopped
     depends_on: