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.

REDEPLOY.md

@@ -9,16 +9,16 @@ Whole thing is ~5 minutes, most of it waiting on `docker build`. Order matters:
 **back up before you pull.** A backup taken after a bad migration is a backup of
 the damage.
 
-Paths below assume the checkout is at `/opt/bookmarkmanager`; substitute your own. The
-one absolute rule about paths: **backups live in `../bookmarkmanager-backups/`**, a
-sibling of the project directory (`/opt/bookmarkmanager-backups`), never inside it. It
+Paths below assume the checkout is at `~/mangaBookmark`, which is where it lives
+on this deployment; substitute your own. The one absolute rule about paths:
+**backups live in a `-backups` sibling of the checkout**, never inside it. It
 sits outside the repo so `git pull`, `git clean -fd` and a bad `rm -rf` inside
 the checkout cannot take the backups with them.
 
 ```
-/opt/
-├── bookmarkmanager/            <- the checkout (this repo)
-└── bookmarkmanager-backups/    <- bookmarks-YYYYmmdd-HHMMSS.dump
+~/
+├── mangaBookmark/            <- the checkout (this repo)
+└── mangaBookmark-backups/    <- bookmarks-YYYYmmdd-HHMMSS.dump
 ```
 
 ---
@@ -26,7 +26,7 @@ the checkout cannot take the backups with them.
 ## 0. Preflight
 
 ```bash
-cd /opt/bookmarkmanager
+cd ~/mangaBookmark
 
 # Both -f flags, every time. The prod override is not standalone.
 COMPOSE="docker compose -f docker-compose.yml -f docker-compose.prod.yml"
@@ -44,9 +44,9 @@ dirty tree fails halfway and leaves you in a worse spot than either.
 Create the backup directory once, and make sure it is a sibling, not a child:
 
 ```bash
-mkdir -p ../bookmarkmanager-backups
-BACKUP_DIR="$(cd .. && pwd)/bookmarkmanager-backups"   # absolute — Docker needs it
-echo "$BACKUP_DIR"                             # -> /opt/bookmarkmanager-backups
+BACKUP_DIR="$(cd .. && pwd)/$(basename "$PWD")-backups"   # absolute — Docker needs it
+mkdir -p "$BACKUP_DIR"
+echo "$BACKUP_DIR"                        # -> /home/sulthan/mangaBookmark-backups
 ```
 
 ---
@@ -131,8 +131,11 @@ container stopped the shutdown checkpoint has already flushed everything and a
 plain archive of the volume is consistent.
 
 ```bash
-VOL=$(docker volume ls --filter name=postgres-data -q | head -1)
-echo "$VOL"                          # -> bookmarkmanager_postgres-data
+# Derived exactly, not with a `--filter name=` substring match plus `head -1`:
+# that quietly picks the first of however many volumes happen to contain the
+# string, and archiving the wrong data directory is not a visible failure.
+VOL="$(basename "$PWD" | tr '[:upper:]' '[:lower:]')_postgres-data"
+docker volume inspect "$VOL" >/dev/null && echo "$VOL"   # -> mangabookmark_postgres-data
 
 $COMPOSE stop
 docker run --rm -v "$VOL":/from:ro -v "$BACKUP_DIR":/to alpine \
@@ -165,10 +168,10 @@ down -v` from taking it with the rest of the stack. It is not the live database
 and nothing reads it — the one-way move out of it is `CUTOVER.md`. Once the
 Postgres data has been trusted for a while, remove it by hand — nothing else will.
 Its full name is `<compose project>_bookmarks-data`, and the project name is the
-lowercased directory name of the checkout, so ask Docker rather than typing it:
+lowercased directory name of the checkout:
 
 ```bash
-docker volume rm "$(docker volume ls -q --filter name=_bookmarks-data)"
+docker volume rm "$(basename "$PWD" | tr '[:upper:]' '[:lower:]')_bookmarks-data"
 ```
 
 ---