feat: DDNS self-healing heartbeat + manual re-register endpoint

- DDNSTokenExpired exception triggers auto re-register in update_ip()
  so cells recover silently after a DDNS DB reset
- POST /api/ddns/register lets the user force re-registration from Settings
- Re-register button in Settings → External Domain & DDNS (pic_ngo only)
- 3 new tests covering register endpoint: wrong provider, missing name, success

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/ddns_manager.py

@@ -37,6 +37,10 @@ class DDNSError(Exception):
     """Raised when a DDNS provider returns an error response."""
 
 
+class DDNSTokenExpired(DDNSError):
+    """Raised when the DDNS service rejects the token (401) — usually after a DB reset."""
+
+
 # ---------------------------------------------------------------------------
 # Provider base class
 # ---------------------------------------------------------------------------
@@ -96,6 +100,10 @@ class PicNgoDDNS(DDNSProvider):
 
     def _raise_for_status(self, response: requests.Response, action: str):
         if not response.ok:
+            if response.status_code == 401:
+                raise DDNSTokenExpired(
+                    f"PicNgoDDNS {action} rejected token: HTTP 401 — {response.text}"
+                )
             raise DDNSError(
                 f"PicNgoDDNS {action} failed: HTTP {response.status_code} — {response.text}"
             )
@@ -432,6 +440,18 @@ class DDNSManager(BaseServiceManager):
                 self._last_ip = current_ip
             else:
                 logger.warning("DDNS update_ip: provider.update() returned False")
+        except DDNSTokenExpired:
+            logger.warning("DDNS update_ip: token rejected (401) — attempting re-registration")
+            try:
+                cell_name = self._identity().get('cell_name', '')
+                if cell_name:
+                    self.register(cell_name, current_ip)
+                    logger.info("DDNS re-registered after token expiry: cell_name=%r", cell_name)
+                    self._last_ip = current_ip
+                else:
+                    logger.error("DDNS update_ip: cannot re-register — cell_name not in identity")
+            except Exception as exc2:
+                logger.error("DDNS update_ip: re-registration failed: %s", exc2)
         except DDNSError as exc:
             logger.error("DDNS update_ip: provider error: %s", exc)