docs(server): add systemd user-service setup for openviking-server

[Gujiassh]

Summary

• add a ready-to-copy systemd --user unit example for openviking-server
• document user-service setup/start/log commands in server quickstart docs
• link server startup guidance in root README to the new systemd section

Problem

Issue #1218 reports that shell background wrappers (like nohup ... &) lead to unstable service lifecycle behavior under systemd.

Why this change

This provides an officially documented, process-manager-native way to run OpenViking in persistent Linux sessions without relying on wrapper/background shell patterns.

Validation

• systemd-analyze verify docs/en/getting-started/examples/openviking-server.service

Closes #1218

[github-actions]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

🎫 Ticket compliance analysis ✅ 1218 - Fully compliant Compliant requirements: Add systemd user service example Document systemd setup in server quickstart Link to systemd docs from root README

⏱️ Estimated effort to review: 2 🔵🔵⚪⚪⚪

🏅 Score: 85

🧪 No relevant tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ Recommended focus areas for review Quoting Issue The ExecStart line uses unquoted ${OPENVIKING_CONFIG_FILE}, which breaks when the config path contains spaces. ExecStart=/usr/bin/env openviking-server --config ${OPENVIKING_CONFIG_FILE}

[github-actions]

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[qin-ctx]

Thanks for the contribution. However, this PR still does not fully resolve issue #1384. The documentation changes need further improvement in both the Chinese and English versions, and the newly added content is not placed in the most appropriate section, which makes the overall structure a bit confusing. Please prioritize updating deployment.md first.