feat(2.0): Phase 5 — Betrieb, Observability + Politur

Backup: services/backup.py (Memory-SQLite inkl. WAL/SHM + Configs, retain 7)
+ deploy/backup.sh + POST /api/system/backup + GET /api/system/backups.
Services-Health: GET /api/system/services (aggregierte Reachability +
Observability-Links). Frontend: SystemView mit Dienste-Health + Backup-
Button + Links; Theme-Toggle (Hell/Dunkel, persistent).

Lokal verifiziert: Backup-Snapshot + services-Aggregat (TestClient),
Frontend-Build + Browser (Light-Mode bestaetigt via computed styles).
Damit Phasen 0-5 lokal fertig. Docs aktualisiert.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -1,5 +1,5 @@
 import { useEffect, useState } from "react"
-import { Command as CommandIcon } from "lucide-react"
+import { Command as CommandIcon, Moon, Sun } from "lucide-react"
 import { NAV, type ViewId } from "@/nav"
 import { CommandPalette } from "@/components/CommandPalette"
 import { ModelsView } from "@/views/ModelsView"
@@ -15,6 +15,7 @@ import { cn } from "@/lib/utils"
 export default function App() {
   const [view, setView] = useState<ViewId>("models")
   const [health, setHealth] = useState<Health | null>(null)
+  const [dark, setDark] = useState(() => localStorage.getItem("mc_theme") !== "light")
 
   useEffect(() => {
     const load = () => api<Health>("/api/health").then(setHealth).catch(() => setHealth(null))
@@ -23,6 +24,11 @@ export default function App() {
     return () => clearInterval(t)
   }, [])
 
+  useEffect(() => {
+    document.documentElement.classList.toggle("dark", dark)
+    localStorage.setItem("mc_theme", dark ? "dark" : "light")
+  }, [dark])
+
   const active = NAV.find((n) => n.id === view)!
 
   return (
@@ -78,6 +84,14 @@ export default function App() {
       <div className="flex min-w-0 flex-1 flex-col">
         <header className="flex h-14 shrink-0 items-center justify-between border-b border-border px-6">
           <div className="text-sm text-muted-foreground">{active.hint}</div>
+          <div className="flex items-center gap-2">
+          <button
+            onClick={() => setDark((d) => !d)}
+            className="flex h-8 w-8 items-center justify-center rounded-md border border-border text-muted-foreground hover:bg-accent"
+            title="Hell/Dunkel"
+          >
+            {dark ? <Sun className="h-3.5 w-3.5" /> : <Moon className="h-3.5 w-3.5" />}
+          </button>
           <button
             onClick={() => {
               const ev = new KeyboardEvent("keydown", { key: "k", metaKey: true })
@@ -89,6 +103,7 @@ export default function App() {
             <span>Springen</span>
             <kbd className="rounded bg-muted px-1.5 py-0.5 font-mono text-[10px]">⌘K</kbd>
           </button>
+          </div>
         </header>
 
         <main className="flex-1 overflow-y-auto p-6">