docs: add iFlow Search OpenAPI server

[zhengyanglsun]

• Adds a minimal iFlow Search OpenAPI server reference under servers/iflow-search.
• Documents the npm-based server path using npx -y @iflow-ai/search-openapi@next.
• Exposes web search, image search, and URL fetch tools through /openapi.json.
• Notes that this server is Node/npm-native, unlike the Python/FastAPI examples in this repository, so this PR intentionally adds README-only guidance rather than main.py / requirements.txt.
• Documents the key boundary: IFLOW_API_KEY stays in the local search-openapi server process and is not sent to Open WebUI.
• Uses placeholders only; no real API keys are included.
• Does not modify compose.yaml or add Docker artifacts.

[zhengyanglsun]

Status update — @iflow-ai/search-openapi@0.1.0 has shipped to npm latest (verifiable with npm view @iflow-ai/search-openapi dist-tags).

The OpenAPI 3.1 surface (/openapi.json), Coze 3.0.3 surface (/openapi.coze.json), three tool routes, and response shapes are unchanged from the prerelease line used to validate this PR.

Validation summary:

• Open WebUI import + tool invocation against /openapi.json was verified on the published prerelease tarball.
• Coze import + Tool Debug + Agent invocation against /openapi.coze.json was verified on the published prerelease tarball.
• The stable 0.1.0 cut keeps the same tool names and request/response shape.

This PR currently mentions @next / prerelease usage. That is still functionally correct because @next remains the prerelease channel, but bare npx -y @iflow-ai/search-openapi now resolves to stable 0.1.0.

Happy to refresh the branch to replace the @next framing with stable install snippets if maintainers prefer. This is a community / third-party integration; no implied Open WebUI endorsement.