Add env-driven SMTP and app identity settings for PocketBase

- pb_hooks/main.pb.js: bootstrap hook applying SMTP + meta settings from
  env vars on every boot (mail disabled when SMTP_HOST is unset)
- docker-compose.yml: expose SMTP_* / PB_APP_URL on the pocketbase service
- pocketbase/Dockerfile: bundle pb_hooks
- README: document the new variables

README.md

@@ -30,6 +30,20 @@ Node >= 22.12.
 | ---------------- | -------------------------------------------- | ------------------------------------ |
 | `POCKETBASE_URL` | URL of the PocketBase backend (auth + data)  | `https://pokeshare.namarusaja.me`    |
 
+PocketBase container (compose only):
+
+| Variable                             | Description                                       |
+| ------------------------------------ | ------------------------------------------------- |
+| `PB_SUPERUSER_EMAIL` / `_PASSWORD`   | Create/update the admin account on boot           |
+| `PB_APP_URL`                         | Public app URL used in transactional email links  |
+| `SMTP_HOST` / `SMTP_PORT`            | Mail server (mail disabled if `SMTP_HOST` empty)  |
+| `SMTP_USERNAME` / `SMTP_PASSWORD`    | Mail server credentials                           |
+| `SMTP_TLS`                           | `true` (default) / `false`                        |
+| `SMTP_SENDER_NAME` / `_ADDRESS`      | From header of transactional emails               |
+
+SMTP settings are applied on every boot by `pocketbase/pb_hooks/main.pb.js`
+from these env vars, so they can be changed without touching the admin UI.
+
 Read at **runtime** (`process.env`), so the same Docker image can be deployed
 against different PocketBase instances. For local dev, copy `.env.example` to
 `.env`. `HOST` and `PORT` are also honored by the standalone server.