feat: add configurable build metadata

[vigneshrajsb]

Summary

• Add global build metadata configuration with ordered link templates.
• Add v2 endpoints for rendered build metadata and admin metadata config maintenance.
• Render metadata links with the existing build environment variable context and validate rendered URLs.
• Use configured metadata links in activity stream dashboard Markdown.
• Remove the unused builds.dashboardLinks column and stop exposing it from the v1 build endpoints.

Notes

• The new rendered metadata endpoint is GET /api/v2/builds/{uuid}/metadata.