Add optional AMap POI service and contributor setup docs

[RCF-117]

Summary

• add docs/CONTRIBUTING.md and align local setup docs around uv, project-local caches, and health-check smoke tests
• add an optional backend amap_service for AMap Web Service POI lookup with TTL caching and stub fallback
• wire candidate POI flows through the new service while preserving the existing no-key stub behavior
• ensure SQLite file database parent directories are created during backend startup

Notes

• AMAP_KEY remains optional. Without it, local development and tests continue to use the existing stub data.
• The AMap client degrades back to stub data on missing keys, request failures, empty responses, or invalid coordinates.
• This keeps the change scoped to contributor setup and the POI service boundary, without requiring a paid service for new contributors.

Validation

• uv run pytest -> 60 passed
• npm test -- --run -> 10 passed
• npm run build -> passed
• FastAPI TestClient /health -> 200 {'status': 'ok'}

[kaikkd]

本地复核通过：后端 60 项测试通过，前端 10 项测试通过，生产构建通过。高德响应解析、无 Key 降级、缓存和 SQLite 父目录处理整体清晰；留一条非阻塞的数据契约建议。

[kaikkd]

[P3] 请把 amap_id 继续透传到前端候选/排程链路

这里开始返回真实高德 ID，但当前 frontend/src/api/client.ts 的 toPoi() 仍固定写入 amap_id: null，SelectedPoiPayload 和后端 SelectedPoi 也没有该字段。因此 /plan/candidates 返回的 ID 在用户选中 POI 后立即丢失，后续反查、去重或 grounding 无法使用这次新增的数据。

这不阻塞当前展示，所以作为非阻塞建议：让 toPoi 使用 p.amap_id ?? null，并在 selected POI payload 与排程转换中继续透传；如果本 PR 有意只负责后端，请至少建立跟踪 issue。