feat: slider pre-flash advance (0-3s) dans settings UI

backend/api/leds_api.py

@@ -158,6 +158,36 @@ async def measure_ambient_lux(request: Request):
         return JSONResponse({"ok": False, "error": str(e)}, status_code=500)
 
 
+@router.get("/leds/flash-timing")
+async def get_flash_timing(request: Request):
+    """Retourne les paramètres de timing du flash."""
+    cfg = request.app.state.config.leds
+    return {
+        "pre_flash_advance": cfg.pre_flash_advance,
+        "countdown_duration": cfg.countdown_duration,
+    }
+
+
+@router.put("/leds/flash-timing")
+async def set_flash_timing(
+    request: Request,
+    pre_flash_advance: float = Query(..., ge=0.0, le=3.0, description="Secondes avant fin countdown pour démarrer le flash"),
+    save: bool = Query(default=True),
+):
+    """Règle combien de secondes avant la fin du countdown le flash démarre (0–3s)."""
+    cfg = request.app.state.config.leds
+    cfg.pre_flash_advance = pre_flash_advance
+    if save:
+        config_svc = request.app.state.config_service
+        with open(config_svc.path, encoding="utf-8") as f:
+            import yaml
+            raw = yaml.safe_load(f) or {}
+        raw.setdefault("leds", {})["pre_flash_advance"] = pre_flash_advance
+        with open(config_svc.path, "w", encoding="utf-8") as f:
+            yaml.dump(raw, f, allow_unicode=True, default_flow_style=False)
+    return {"ok": True, "pre_flash_advance": pre_flash_advance}
+
+
 @router.put("/leds/effect/{effect_name}")
 async def update_effect(
     request: Request,