fix(Docker): require persistent DATA_DIR volume; add VOLUME + README data-persistence warning

Root cause of re-login failing after first-run setup: on EasyPanel/w/o a volume, the
container's /app/backend/data (users, incl. the changed admin password) is wiped on every
recreate, so logging back in with the new password fails. Added:
- ENV DATA_DIR=/app/backend/data (explicit) + VOLUME declaration in Dockerfile
- README 'Data persistence' section: must mount a persistent volume to /app/backend/data

Verified: backend + live server login>setup>relogin-with-new-password works (200); the
failure is deployment data persistence, not login logic.

Dockerfile

@@ -28,6 +28,12 @@ COPY backend/ ./backend/
 
 # Run
 WORKDIR /app/backend
+# Explicit data dir so operators mount a persistent volume here.
+# Without a volume, ALL user data (incl. the admin password change from first-run
+# setup) is lost on every container recreate -> re-login with the new password fails.
+# Persist with:  docker run -v ./data:/app/backend/data  (or compose ./data)
+ENV DATA_DIR=/app/backend/data
 ENV FLASK_DEBUG=0
 EXPOSE 5001
+VOLUME ["/app/backend/data"]
 CMD ["python", "run.py"]