feat: 大屏后端（FastAPI+MQTT+sim_engine）

- REST :10085 / MQTT 控制 / SSE 回退 / AI+语音
- 数据源规划：接核心服务端 + 算力引擎真实数据

backend/app/vision_llm.py

@@ -0,0 +1,83 @@
+# -*- coding: utf-8 -*-
+"""对话开场画面识别 —— 用多模态 LLM（qwen3-vl-flash）识别实时画面中的人数、性别等，
+并把结果作为上下文注入对话，让 AI 了解当前在场观众。
+
+与 vision_yolo（YOLO 数人脸/姿态）互补：这里用 LLM 做语义级理解（人数、性别构成、场景描述）。
+"""
+import json
+import logging
+import ssl
+import urllib.request
+
+import certifi
+
+from .config import settings
+
+log = logging.getLogger("dpm.vision")
+
+_SSL_CTX = ssl.create_default_context(cafile=certifi.where())
+
+_DEFAULT_PROMPT = (
+    "请识别这张实时画面，只输出一个 JSON 对象（不要输出任何其他文字）："
+    '{"people": 画面中人数(int), "males": 其中男性人数(int), "females": 其中女性人数(int), '
+    '"desc": 一句话中文描述画面（含大致人数、性别构成、人物大致状态，如年龄/坐站/是否看屏幕）。'
+    "若画面无人或不确定，则 people=0、males=0、females=0，desc='画面中暂时没有人'。"
+)
+
+
+def analyze_scene(jpeg_b64: str, prompt: str = "") -> dict:
+    """调用多模态 LLM 识别画面中人数/性别，返回 {ok, people, males, females, desc}。"""
+    payload = {
+        "model": settings.VISION_LLM_MODEL,
+        "messages": [{
+            "role": "user",
+            "content": [
+                {"type": "text", "text": prompt or _DEFAULT_PROMPT},
+                {"type": "image_url", "image_url": {"url": f"data:image/jpeg;base64,{jpeg_b64}"}},
+            ],
+        }],
+    }
+    req = urllib.request.Request(
+        f"{settings.LLM_BASE_URL}/chat/completions",
+        data=json.dumps(payload, ensure_ascii=False).encode("utf-8"),
+        headers={
+            "Content-Type": "application/json",
+            "Authorization": f"Bearer {settings.DASHSCOPE_API_KEY}",
+        },
+    )
+    with urllib.request.urlopen(req, timeout=30, context=_SSL_CTX) as resp:
+        data = json.loads(resp.read().decode("utf-8"))
+    content = data["choices"][0]["message"].get("content") or ""
+    return _parse(content)
+
+
+def _parse(text: str) -> dict:
+    """从模型输出中抽取 JSON 对象并规范化为结果 dict。"""
+    text = (text or "").strip()
+    if text.startswith("```"):
+        lines = [l for l in text.splitlines() if not l.strip().startswith("```")]
+        text = "\n".join(lines).strip()
+    obj: dict = {}
+    i, j = text.find("{"), text.rfind("}")
+    if i != -1 and j > i:
+        try:
+            obj = json.loads(text[i:j + 1])
+        except Exception:  # noqa: BLE001
+            log.warning("vision llm: 返回 JSON 解析失败: %s", text[:120])
+    people = _as_int(obj.get("people"))
+    males = _as_int(obj.get("males"))
+    females = _as_int(obj.get("females"))
+    return {
+        "ok": True,
+        "people": people,
+        "males": males,
+        "females": females,
+        "desc": str(obj.get("desc") or "").strip(),
+    }
+
+
+def _as_int(v):
+    try:
+        return int(float(v))
+    except (TypeError, ValueError):
+        return 0