Share accounts: per-site Samba passwords from the webapp

Tools > Share accounts sets the pxe-upload, blancco and clonezilla Samba
passwords. Saving one runs smbpasswd and writes it everywhere it is used:
startnet.cmd in boot.wim (pxe-upload), the served Blancco preferences and
the live config.img (blancco), the iPXE menu's Clonezilla entries
(clonezilla). Refused while a PC is imaging unless forced. Stored 0600 in
UPDATES_DIR/shares/accounts.json, never shown back.

- The stored values are the authority: the playbook only creates the
  Samba accounts, seeds the store from site-vars (pxe_upload_password,
  blancco_smb_password, clonezilla_smb_password) for accounts it does not
  hold, and runs "share_accounts apply" at the end of every run.
- Every startnet save re-pins SHAREPASS, so a restored snapshot cannot
  bring an old password back; new menu blocks use %SHAREUSER%/%SHAREPASS%.
- Shop floor unattend Order 4 reads the login from fetch-source.txt;
  Invoke-PostPpkg deletes that file once the PPKG is applied.
- Upload-Image.ps1 asks for the password unless -Pass is given.
- pxe_admin_password: the Linux pxe login. Required while the box still
  has the shipped "pxe"; set with chpasswd when it differs.
- Blancco site settings now also reach the live config.img (BIOS path).

webapp/views/blancco_update.py

@@ -45,14 +45,16 @@ def page():
 @bp.route("/site-settings", methods=["POST"])
 def site_settings():
     """Business location and licensing Wi-Fi: saved, then the served
-    preferences.xml is rendered again. The password is never shown back."""
+    preferences.xml and the live config.img are written again. The password
+    is never shown back."""
     try:
         with bu._lock:
             blancco_prefs.save(request.form.get("business_location", ""),
                                request.form.get("wifi_ssid", ""),
                                request.form.get("wifi_password", ""))
-            changed = blancco_prefs.render()
-    except (OSError, ValueError) as e:
+        # Served preferences.xml and the live config.img (BIOS path).
+        changed = bu.refresh_live_prefs()
+    except (OSError, ValueError, bu.StageBlocked) as e:
         flash(f"Blancco site settings not saved: {e}", "danger")
         return _back()
     s = blancco_prefs.public()