docs: record login-after-setup root cause (deployment data persistence)

docs/engineering-log/2026-08-07-login-after-setup.md

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+# 2026-08-07 — Bug: can't log in after first-run setup (password wrong) — root cause + fix
+
+## Symptom
+After first login (admin/1234) → change password in the forced setup screen → logout → logging
+back in with the new password reports "wrong password".
+
+## Investigation (verified via reproduction)
+1. Backend `repro_login` in a temp data dir: first login (200, must_setup=true) → setup
+   (200, must_setup=false, email set) → **re-login with NEW password = 200** (works) → old
+   password = 401. => Login logic is CORRECT.
+2. Same flow over the LIVE localhost server via HTTP (created tester2, setup with new pw,
+   re-login with new pw = 200). => Works end-to-end on a running server.
+3. Conclusion: **not a login/setup code bug.** The failure is DEPLOYMENT DATA NON-PERSISTENCE.
+
+## Root cause
+On EasyPanel / any deploy that doesn't mount a volume, the container's `DATA_DIR`
+(`/app/backend/data`) is **ephemeral — wiped on every container recreate/restart**. So after
+setup, when the container is recreated:
+- the changed admin password is lost (record resets to `1234` / `must_setup=true`),
+- logging back in with the new password fails ("รหัสผ่านผิด").
+
+## Fix
+- Dockerfile: `ENV DATA_DIR=/app/backend/data` (explicit) + `VOLUME ["/app/backend/data"]`.
+- README: added a "Data persistence" section — must mount a persistent volume to
+  `/app/backend/data` (compose already does `./data:/app/backend/data`); EasyPanel must mount one.
+- Commit `334ef4e` (+ earlier docker commit), pushed to `git.moreminimore.com/kunthawat/sales-trainer`.
+
+## How the user should apply
+- **EasyPanel**: add a persistent volume mount → `/app/backend/data` (or set `DATA_DIR` env to a
+  mounted path). After that, data survives redeploys.
+- If they already lost data (old password gone): delete/reset the container data so a fresh
+  `admin`/`1234` is bootstrapped, then redo setup and NEITHER redeploy without a volume.
+
+## Note
+Local dev: `backend/data/` is gitignored and lives on disk, so this only bites container deploys.