fix: make sure smtp can be set either via env or ui

docs/smtp-configuration-concept.md

@@ -25,7 +25,10 @@ Enable configurable SMTP for sending transactional emails (join confirmation, us
 | ENV      | 1        | Production, Docker, 12-factor     |
 | Settings | 2        | Admin UI, dev without ENV          |
 
-When an ENV variable is set, the corresponding Settings field is read-only in the UI (with hint "Set by environment").
+When `SMTP_HOST` is set, SMTP runs in **ENV-only mode**:
+- all SMTP fields in Settings are read-only,
+- saving SMTP settings in the UI is disabled,
+- and the UI shows a warning block if required SMTP ENV values are missing.
 
 ---
 
@@ -63,6 +66,14 @@ Support **SMTP_PASSWORD_FILE** (path to file containing the password), same patt
   - Show a warning in the Settings UI.
   - Delivery attempts silently fall back to the Local adapter (no crash).
 
+### 6.1 Behaviour in ENV-only mode (`SMTP_HOST` set)
+
+- The SMTP source of truth is environment variables only.
+- The UI does not allow editing SMTP fields in this mode.
+- The Settings page shows a warning block when required values are missing:
+  - `SMTP_USERNAME`
+  - `SMTP_PASSWORD` or `SMTP_PASSWORD_FILE`
+
 ---
 
 ## 7. Test Email (Settings UI)