docs: close the environment contract and de-ambiguate volume derivation (#26)

Review findings on the previous commit:

- README's config table listed 8 of the backend's 25 environment variables,
  omitting the whole DISCORD_* set that the backend refuses to start without.
  The canonical reference cannot be missing the vars that gate startup.
- `docker volume ls --filter name=` is a substring match. Both runbooks used
  it to find a volume they then mount read-only or delete; a second matching
  volume makes the mount fail obscurely and the delete take both. Derive the
  name exactly from the compose project instead.
- CUTOVER §6 removes the retired volume 'once the Postgres data has been
  trusted for a while', in a shell where §1's $VOL no longer exists.
- REDEPLOY and DEPLOY still pointed at /opt/bookmarkmanager, so CUTOVER §6's
  handoff to REDEPLOY §1 sent the operator to a path that does not exist.

CUTOVER.md

@@ -32,10 +32,10 @@ is dropped) — writing it from the spec below costs less than maintaining it
 would.
 
 Beyond `DEPLOY.md`'s prerequisites (Docker and Compose), this runbook needs
-`python3` — its stdlib `sqlite3` module is the whole SQLite dependency, and it
-stands in for `jq` in §5, which is not installed on the server. It does not have
-to run on the server: §3 only reads the snapshot copy, so it can run on a laptop
-and the resulting `import.sql` be copied over.
+`python3`: its stdlib `sqlite3` module is the whole SQLite dependency, and §5's
+read-path check uses it in place of `jq`, which the server does not have. It
+does not have to run on the server — §3 only reads the snapshot copy, so it can
+run on a laptop and the resulting `import.sql` be copied over.
 
 ---
 
@@ -51,9 +51,12 @@ BACKUP_DIR="$(cd .. && pwd)/$(basename "$PWD")-backups"; mkdir -p "$BACKUP_DIR"
 STAMP=$(date -u +%Y%m%d-%H%M%S)
 
 # The volume is <compose project>_bookmarks-data, and the project name defaults
-# to the lowercased *directory* name, not the repo name — here that makes it
-# mangabookmark_bookmarks-data. Ask Docker instead of typing it out.
-VOL=$(docker volume ls -q --filter name=_bookmarks-data); echo "$VOL"
+# to the lowercased *directory* name, not the repo name — on this host the
+# checkout is ~/mangaBookmark, so the volume is mangabookmark_bookmarks-data.
+# Derive it exactly rather than with a `--filter name=` substring match, which
+# would return every volume whose name merely contains the string.
+VOL="$(basename "$PWD" | tr '[:upper:]' '[:lower:]')_bookmarks-data"
+docker volume inspect "$VOL" >/dev/null && echo "$VOL"
 
 $COMPOSE stop bookmark-api
 
@@ -279,8 +282,10 @@ curl -s -H "Authorization: Bearer $TOKEN" $API/bookmarks |
 
 - **Keep the old SQLite volume for a month.** It is already undeclared in
   compose, so `docker compose down -v` cannot take it. Remove it by hand once
-  the Postgres data has been trusted for a while:
-  `docker volume rm "$VOL"` (see `REDEPLOY.md` §1).
+  the Postgres data has been trusted for a while. That happens in a shell where
+  `$VOL` from §1 is long gone, so re-derive it:
+  `docker volume rm "$(basename ~/mangaBookmark | tr '[:upper:]' '[:lower:]')_bookmarks-data"`
+  (see `REDEPLOY.md` §1).
 - **Delete the generator and the working copies:** `rm -rf /tmp/cutover`. The
   timestamped export in `$BACKUP_DIR` is the copy that is kept.
 - **Take the first Postgres dump immediately** — `REDEPLOY.md` §1. Until that