(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/self_check.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 import os
 import shutil
 import subprocess
+import sys
 from dataclasses import dataclass
 from pathlib import Path
 from typing import Literal
@@ -125,7 +126,7 @@ def _path_check(
 
 def _binary_checks() -> list[SelfCheck]:
     checks = []
-    for binary in ("rsync", "ssh", "ssh-keygen", "gunicorn"):
+    for binary in ("rsync", "ssh", "ssh-keygen"):
         path = shutil.which(binary)
         checks.append(
             SelfCheck(
@@ -134,6 +135,14 @@ def _binary_checks() -> list[SelfCheck]:
                 path or f"{binary} was not found in PATH.",
             )
         )
+    gunicorn_path = shutil.which("gunicorn") or Path(sys.executable).parent / "gunicorn"
+    checks.append(
+        SelfCheck(
+            "Binary: gunicorn",
+            "ok" if Path(gunicorn_path).exists() else "failed",
+            str(gunicorn_path) if Path(gunicorn_path).exists() else "gunicorn was not found in PATH or next to Python.",
+        )
+    )
     return checks