fix: Hostnamen normalisieren (Schema/Slash strippen) + freundlichere Nextcloud-Fehler

Bug: nextcloudhost mit https://-Präfix in CSV/GUI führte zu Doppel-Schema
(https://https://cloud.foo.de) und damit zu DNS-Fehler "host=https".

- models.clean_host() entfernt http(s):// und Trailing-Slash; Account
  ruft das im __post_init__ für plesk-/kerio-/nextcloudhost auf.
- NextcloudClient wickelt requests-Connection-Fehler in NextcloudError
  ein, damit die Log-Ausgabe lesbar bleibt statt HTTPSConnectionPool-Stacktrace.
- README: Hostname-Eingabe als tolerant dokumentiert; pleskhost-Pflicht
  im manual-Modus klarer (POP3-Sammler braucht es trotzdem).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

clients/nextcloud.py

@@ -31,8 +31,17 @@ class NextcloudClient:
             raise NextcloudError(f"Nextcloud {op}: ungültige Antwort: {r.text[:300]}")
         return data.get("ocs", {})
 
+    def _request(self, method: str, url: str, op: str, **kw):
+        try:
+            return self.session.request(method, url, timeout=30, **kw)
+        except requests.RequestException as e:
+            raise NextcloudError(
+                f"Nextcloud {op} – Verbindung zu {url} fehlgeschlagen: {e}"
+            ) from e
+
     def user_exists(self, userid: str) -> bool:
-        r = self.session.get(f"{self.base}/ocs/v2.php/cloud/users/{userid}")
+        r = self._request("GET", f"{self.base}/ocs/v2.php/cloud/users/{userid}",
+                          op="user-lookup")
         ocs = self._meta(r, "user-lookup")
         sc = ocs.get("meta", {}).get("statuscode")
         return sc in (100, 200)
@@ -40,10 +49,8 @@ class NextcloudClient:
     def ensure_group(self, group: str) -> None:
         if not group:
             return
-        r = self.session.post(
-            f"{self.base}/ocs/v2.php/cloud/groups",
-            data={"groupid": group},
-        )
+        r = self._request("POST", f"{self.base}/ocs/v2.php/cloud/groups",
+                          op="group-create", data={"groupid": group})
         ocs = self._meta(r, "group-create")
         sc = ocs.get("meta", {}).get("statuscode")
         # 100/200 = ok, 102 = exists already
@@ -66,10 +73,8 @@ class NextcloudClient:
             body.append(("groups[]", group))
         body.append(("quota", f"{quota_gb} GB" if quota_gb else "none"))
 
-        r = self.session.post(
-            f"{self.base}/ocs/v2.php/cloud/users",
-            data=body,
-        )
+        r = self._request("POST", f"{self.base}/ocs/v2.php/cloud/users",
+                          op="user-create", data=body)
         ocs = self._meta(r, "user-create")
         sc = ocs.get("meta", {}).get("statuscode")
         if sc not in (100, 200):