feat(api): A3 backend skeleton with FastAPI + SQLModel

- 3 tables: Meeting / TranscriptSegment / Summary (with state machine)
- Routes: /api/upload-url + /api/upload-complete + meetings CRUD
- MinIO presigned PUT for direct browser upload
- BackgroundTasks state-machine stub for A5 to flesh out
- SQLite for local dev, PostgreSQL+asyncpg for prod
- CORS configured for frontend on 4490

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/README.md

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+# MeetNote API
+
+FastAPI 后端，端口 4491。
+
+## 启动
+
+```bash
+python3 -m venv .venv
+source .venv/bin/activate
+pip install -r requirements.txt
+cp .env.example .env  # 填 GROQ_API_KEY / POE_API_KEY
+uvicorn app.main:app --reload --port 4491
+```
+
+## 路由
+
+- `GET  /health`
+- `POST /api/upload-url` — 申请 presigned URL 创建会议
+- `POST /api/upload-complete` — 通知后端上传完成，触发后台处理
+- `GET  /api/meetings` — 会议列表
+- `GET  /api/meetings/{id}` — 单个会议
+- `GET  /api/meetings/{id}/transcript`
+- `GET  /api/meetings/{id}/summary`
+- `DELETE /api/meetings/{id}`
+
+## 数据库
+- 默认 `sqlite+aiosqlite:///./meetnote.db`（本地开发）
+- 生产改 `DATABASE_URL=postgresql+asyncpg://...`
+
+## 状态机
+`pending → uploading → uploaded → splitting → transcribing → summarizing → done | failed`
+
+## A3 vs A5
+A3（本提交）只搭好骨架 + state-machine stub。A5 会接上：
+- ffmpeg silencedetect 切片
+- Groq Whisper 真实转写
+- Poe Claude 真实总结