(feature) Add host doctor checks and Django log viewer

Add host-level checks for address, enabled state, SSH credential
selection, and backup directory readiness, and show them on the host
detail page.

Create host backup directories during host creation and prefill new
hosts from the default global config.

Add a staff-only logs view backed by journalctl with filtering by
pobsync unit, priority, and message text.

Improve runtime checks for gunicorn in virtualenv installs and ensure
the native installer grants the service user access to the backup root.

src/pobsync_backend/host_ops.py

@@ -0,0 +1,69 @@
+from __future__ import annotations
+
+import os
+from pathlib import Path
+
+from pobsync.snapshot_meta import resolve_host_root
+
+from .models import GlobalConfig, HostConfig
+from .self_check import SelfCheck
+
+
+HOST_BACKUP_SUBDIRS = ("scheduled", "manual", ".incomplete")
+
+
+def ensure_host_directories(host: HostConfig, global_config: GlobalConfig | None = None) -> Path:
+    global_config = global_config or GlobalConfig.objects.get(name="default")
+    host_root = resolve_host_root(global_config.backup_root, host.host)
+    for subdir in HOST_BACKUP_SUBDIRS:
+        (host_root / subdir).mkdir(parents=True, exist_ok=True)
+    return host_root
+
+
+def collect_host_checks(host: HostConfig, global_config: GlobalConfig | None = None) -> list[SelfCheck]:
+    checks: list[SelfCheck] = []
+    try:
+        global_config = global_config or GlobalConfig.objects.get(name="default")
+    except GlobalConfig.DoesNotExist:
+        return [SelfCheck("Host global config", "failed", "Default global config does not exist.")]
+
+    checks.append(
+        SelfCheck(
+            "Host enabled",
+            "ok" if host.enabled else "warning",
+            "Host is enabled." if host.enabled else "Host is disabled.",
+        )
+    )
+    checks.append(
+        SelfCheck(
+            "Host address",
+            "ok" if host.address.strip() else "failed",
+            host.address.strip() or "Host address is empty.",
+        )
+    )
+
+    credential = host.ssh_credential or global_config.default_ssh_credential
+    checks.append(
+        SelfCheck(
+            "Host SSH credential",
+            "ok" if credential else "warning",
+            str(credential) if credential else "No host or global SSH credential selected.",
+        )
+    )
+
+    host_root = resolve_host_root(global_config.backup_root, host.host)
+    checks.append(_host_path_check("Host backup root", host_root, must_exist=True, must_be_writable=True))
+    for subdir in HOST_BACKUP_SUBDIRS:
+        checks.append(_host_path_check(f"Host directory: {subdir}", host_root / subdir, must_exist=True, must_be_writable=True))
+    return checks
+
+
+def _host_path_check(name: str, path: Path, *, must_exist: bool, must_be_writable: bool) -> SelfCheck:
+    if must_exist and not path.exists():
+        return SelfCheck(name, "failed", f"{path} does not exist.")
+    target = path if path.exists() else path.parent
+    if not target.exists():
+        return SelfCheck(name, "failed", f"{target} does not exist.")
+    if must_be_writable and not os.access(target, os.W_OK):
+        return SelfCheck(name, "failed", f"{target} is not writable by this process.")
+    return SelfCheck(name, "ok", str(path))