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.

DEPLOY.md

@@ -11,7 +11,7 @@ ACME/cert resolver, and control a domain.
 - Docker + Docker Compose on the server.
 - A Traefik instance watching a Docker network (default name assumed: `proxy`).
 - DNS: an `A`/`AAAA` record for `bookmark-api.<yourdomain>` pointing at the server.
-- The repo copied to the server, e.g. `/opt/bookmarkmanager/` (needs `backend/`,
+- The repo copied to the server, e.g. `~/mangaBookmark/` (needs `backend/`,
   `docker-compose.yml`, `docker-compose.prod.yml`, `.env.example`).
 
 Confirm the Traefik network exists (create if not):
@@ -25,7 +25,7 @@ docker network ls | grep proxy || docker network create proxy
 ## 1. Configure `.env`
 
 ```bash
-cd /opt/bookmarkmanager
+cd ~/mangaBookmark
 cp .env.example .env
 ```