feat(backlinks): badge embed snippets + /partners page

[Flotapponnier]

Summary

Two backlink-friendly additions:

• /api/badge///snippet?format=markdown|html|url|json returns a copy-paste embed string for the live SVG badge. Forwards optional chain/region/kind scope to both badge and bench-link URLs.
• /partners page documents every backlink-friendly asset OCB ships (badges, share cards, citation API, OpenAPI, MCP, llms.txt) plus the CC-BY-4.0 license. Reduces friction for providers and journalists who want to integrate or cite OCB.
• /partners added to sitemap.

No behavior change to the existing /api/badge SVG route.

Test plan

• /api/badge/rpc-capabilities/helius/snippet returns markdown snippet
• /api/badge/rpc-capabilities/helius/snippet?format=json returns the JSON envelope with all 3 snippet shapes
• /partners renders, all code blocks copy cleanly, every link resolves