(feature) Add full native installer and self-check page

Expand the systemd installer so it can perform a complete native
installation with sensible defaults: copy the checkout into the target
app directory, create runtime directories, write the environment file,
install dependencies, configure systemd units, and optionally configure
nginx.

Add a staff-only Django self-check page that verifies runtime settings,
required binaries, writable paths, database connectivity, global config
state, and systemd service status when available.

Document installer overrides and expose the self-check from the main
navigation.

src/pobsync_backend/views.py

@@ -24,6 +24,7 @@ from .forms import (
 )
 from .models import BackupRun, GlobalConfig, HostConfig, ScheduleConfig, SnapshotRecord, SshCredential
 from .retention import run_sql_retention_apply, run_sql_retention_plan
+from .self_check import collect_self_checks, summarize_self_checks
 from .snapshot_discovery import discover_snapshots, inspect_snapshot_discovery
 
 
@@ -58,6 +59,19 @@ def dashboard(request):
     return render(request, "pobsync_backend/dashboard.html", context)
 
 
+@staff_member_required
+def self_check(request):
+    checks = collect_self_checks()
+    return render(
+        request,
+        "pobsync_backend/self_check.html",
+        {
+            "checks": checks,
+            "summary": summarize_self_checks(checks),
+        },
+    )
+
+
 @staff_member_required
 def ssh_credentials(request):
     context = {