feat: Resilienz (Fallback-Ketten) und Metriken (#5)

- Fallback-Provider (app/providers/fallback.py) fuer STT/LLM/TTS: Provider-Kette
  der Reihe nach; Config *_FALLBACK; build_orchestrator baut Ketten (dedupliziert)
- LLM-Stream-Fallback nur solange kein Token gesendet wurde
- Metriken (app/metrics.py): In-Memory Counter/Timer, keine externe Dependency
- HTTP-Middleware (Requests/Latenz/Status je Pfad); Pipeline-Stufen-Timing stt/llm/tts;
  Fallback-/Fehlerzaehler; GET /api/metrics (JSON + Prometheus)
- Tests: 58 gruen (+6); Doku aktualisiert (README, BEDIENUNGSANLEITUNG, Architektur, .env.example)

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

app/main.py

@@ -1,4 +1,8 @@
-from fastapi import FastAPI
+import time
+
+from fastapi import FastAPI, Request
+
+from app.metrics import metrics
 from app.api.health import router as health_router
 from app.api.chat import router as chat_router
 from app.api.transcribe import router as transcribe_router
@@ -8,9 +12,27 @@ from app.api.sessions import router as sessions_router
 from app.api.config import router as config_router
 from app.api.admin import router as admin_router
 from app.api.me import router as me_router
+from app.api.metrics import router as metrics_router
 from app.api.ws import router as ws_router
 
 app = FastAPI(title="Voice Assistant Gateway")
+
+
+@app.middleware("http")
+async def record_metrics(request: Request, call_next):
+    start = time.perf_counter()
+    response = await call_next(request)
+    duration = time.perf_counter() - start
+    # Route-Template (z. B. /api/sessions/{session_id}/route) statt konkreter URL,
+    # um die Label-Kardinalitaet niedrig zu halten.
+    route = request.scope.get("route")
+    path = getattr(route, "path", request.url.path)
+    labels = {"method": request.method, "path": path}
+    metrics.inc("http_requests_total", {**labels, "status": response.status_code})
+    metrics.observe("http_request_duration_seconds", duration, labels)
+    return response
+
+
 app.include_router(health_router)
 app.include_router(chat_router, prefix="/api")
 app.include_router(transcribe_router, prefix="/api")
@@ -20,4 +42,5 @@ app.include_router(sessions_router, prefix="/api")
 app.include_router(config_router, prefix="/api")
 app.include_router(admin_router, prefix="/api")
 app.include_router(me_router, prefix="/api")
+app.include_router(metrics_router, prefix="/api")
 app.include_router(ws_router)