docs: 交接文档三件套(接手总览+后端地图+差距路线图)+补全env模板

- HANDOVER.md: 守印接手第一入口,全景+上手路径+文档地图
- backend/BACKEND_MAP.md: 后端接手地图(路由/模型/AI引擎/评分维度)
- 路线图.md: 现状vs目标差距(三视角×双目标矩阵+7/15倒排冲刺)
- .env.example x3: 补全 MODEL_VISION/CLAUDE_API_URL 及本地SSE示例

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/.env.example

@@ -2,3 +2,8 @@
 # These are build-time variables. After changing them, rebuild the frontend image.
 NEXT_PUBLIC_API_BASE_URL=
 NEXT_PUBLIC_SSE_BASE_URL=
+
+# Local development: the frontend runs on :3000 and the backend on :8000, so
+# SSE cannot use Next.js dev proxy (it buffers EventSource). Set this explicitly:
+#   NEXT_PUBLIC_SSE_BASE_URL=http://localhost:8000
+# and optionally NEXT_PUBLIC_API_BASE_URL=http://localhost:8000 in .env.local