Fix: ZFS-Fehlerlogs auf LXC/non-ZFS-Systemen unterdrückt

- ZFS_AVAILABLE Flag beim Import gesetzt (shutil.which)
- FileNotFoundError in run_command: ERROR → DEBUG
- list_pools/get_pool_status/list_datasets/list_snapshots: frühzeitiger
  Return wenn kein zpool vorhanden → keine nutzlosen Subprocess-Aufrufe

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

backend/services/zfs_runner.py

@@ -15,6 +15,10 @@ import re
 
 logger = logging.getLogger(__name__)
 
+# Detect ZFS availability once at import time — avoids repeated ERROR logs on LXC/non-ZFS systems
+import shutil as _shutil
+ZFS_AVAILABLE = bool(_shutil.which("zpool"))
+
 # Cache with TTL
 @dataclass
 class CacheEntry:
@@ -83,7 +87,7 @@ class ZFSRunner:
             logger.error(f"Command timeout after {timeout}s: {' '.join(cmd)}")
             return "", f"Command timeout after {timeout}s", -1
         except FileNotFoundError:
-            logger.error(f"Command not found: {' '.join(cmd)}")
+            logger.debug(f"Command not found: {' '.join(cmd)}")
             return "", f"Command not found: {cmd[0]}", -1
         except Exception as e:
             logger.error(f"Command execution error: {e}")
@@ -100,6 +104,9 @@ class ZFSRunner:
         if cached:
             return cached
 
+        if not ZFS_AVAILABLE:
+            return []
+
         stdout, stderr, rc = self.run_command(["zpool", "list", "-H", "-p"])
 
         if rc != 0:
@@ -266,6 +273,9 @@ class ZFSRunner:
         """
         Get detailed pool status including VDEV tree and error counters
         """
+        if not ZFS_AVAILABLE:
+            return {}
+
         stdout, stderr, rc = self.run_command(["zpool", "status", pool_name])
 
         if rc != 0:
@@ -347,6 +357,9 @@ class ZFSRunner:
         if cached and cached.get(pool_name):
             return cached[pool_name]
 
+        if not ZFS_AVAILABLE:
+            return []
+
         stdout, stderr, rc = self.run_command([
             "zfs", "list", "-d", str(max_depth), "-H", "-p",
             "-o", "name,used,avail,refer,mountpoint,type",
@@ -446,6 +459,9 @@ class ZFSRunner:
         if cached:
             return cached
 
+        if not ZFS_AVAILABLE:
+            return []
+
         if dataset_name:
             # No -d limit so sub-datasets (e.g. tank/share) are included
             cmd = ["zfs", "list", "-t", "snapshot", "-r", "-H", "-p",