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

pocketbase/Dockerfile

@@ -12,6 +12,7 @@ RUN curl -fsSL "https://github.com/pocketbase/pocketbase/releases/download/v${PB
     && rm pb.zip
 
 COPY pb_migrations ./pb_migrations
+COPY pb_hooks ./pb_hooks
 COPY entrypoint.sh ./
 RUN chmod +x entrypoint.sh pocketbase