Supprimer tout fallback webcam/simulation quand le DSLR est absent

- Preview gphoto2 echoue -> None direct, pas de webcam ni simulation
- Connexion sans DSLR -> mode erreur, camera non connectee
- Suppression _preview_webcam_fallback et _init_preview_fallback
- WS preview : appel direct camera.preview(), plus de logique speciale

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/main.py

@@ -635,15 +635,7 @@ async def traiter_message_ws(msg: dict, ws: WebSocket):
     elif type_msg == "preview":
         import time as _time
         global _dernier_envoi_erreur_preview
-        # Pendant une capture DSLR, utiliser directement la webcam fallback
-        donnees = None
-        if capture_en_cours and camera._preview_webcam_fallback:
-            ret, frame = camera._preview_webcam_fallback.read()
-            if ret:
-                _, buf = cv2.imencode(".jpg", frame, [cv2.IMWRITE_JPEG_QUALITY, 70])
-                donnees = buf.tobytes()
-        else:
-            donnees = camera.preview()
+        donnees = camera.preview()
         if donnees:
             b64 = base64.b64encode(donnees).decode("ascii")
             await ws.send_json({"type": "preview", "image": f"data:image/jpeg;base64,{b64}"})