feat(deploy): make the all-in-one image installable without a shell (#1124)

The all-in-one image could not be installed from a GUI at all — the deployment it
exists for. validateEnv treats a missing JWT_SECRET as critical and exits, and the
documented run command supplies it with `openssl rand`, a shell command a Synology
Container Manager or QNAP Container Station form cannot run.

wait-for-db.sh now generates one on first start and persists it next to the database,
extending the existing /run/secrets hydration rather than adding a second mechanism.
Explicit env still wins, then /run/secrets, then the generated file. The write is
load-bearing: JWT_SECRET is exported only when the file actually persisted, because an
unpersisted secret would mint a new one every restart and sign every session out.

Creation writes to a private temp file and hard-links it into place — atomic, fails with
EEXIST when another container won, and the loser adopts the winner's value. Non-regular
paths are rejected before the link, since POSIX ln links INTO a directory rather than
failing, which would make a mistyped -v target unrecoverable.

Also repairs the onboarding paths a new install actually walks: the installer no longer
rotates the secrets of a running install on re-run, deprecates the dead scripts/install.sh
in place, corrects the CONTRIBUTING dev loop, and fixes the vite proxy target that had
been pointing at a stray local port since 0da45e69.

Reviewed over three rounds. Co-authored by @Luca-Timo.

README.md

@@ -75,13 +75,14 @@ For a home server, a NAS, or a single small studio, the all-in-one image runs th
 ```bash
 docker run -d --name picpeak -p 3000:3000 \
   -v picpeak:/data \
-  -e JWT_SECRET="$(openssl rand -base64 48)" \
   ghcr.io/picpeak/picpeak/aio:main
 ```
 
-Then open **http://localhost:3000/admin** and read the setup token with `docker exec picpeak cat /data/db/SETUP_TOKEN`.
+No environment variables to set — the JWT secret is generated on first start and kept on the volume.
 
-`:main` is the active-development tag, and today it is the only one the all-in-one image has — `Dockerfile.aio` landed after the current stable release, so `:stable` and `:latest` first appear for this image once the aio build reaches the `stable` branch. Switch to `:stable` then, or pin a version tag (`3.107.4-beta.0`) if you would rather not track `main`.
+Then open **http://localhost:3000/admin** and read the setup token with `docker exec picpeak cat /data/db/SETUP_TOKEN`, or open `db/SETUP_TOKEN` on the volume with any file manager if the host has no shell.
+
+`:main` is the active-development tag, and today it is the only one the all-in-one image has — `Dockerfile.aio` landed after the current stable release, so `:stable` and `:latest` first appear for this image once the aio build reaches the `stable` branch. Switch to `:stable` then, or pin a published version tag if you would rather not track `main`.
 
 The compose stack above is still the right choice for anything busier — SQLite takes one writer at a time, and Postgres is what scales. You can move to it later without reinstalling: take a `.picpeak` backup and restore it into the full stack. See **[Single-container install](https://docs.picpeak.app/deployment/single-container)** for the volume layout, the external-Postgres variant, TLS, and the limits.