feat: add Exa AI-powered search tracking header

[tgonzalezc5]

Summary

• Adds x-exa-integration: prismercloud header to both Exa API clients (/api/search and /api/content)
• This allows Exa to attribute API usage to the PrismerCloud integration for analytics and partnership tracking
• Zero behavior change; only adds one header per client instantiation

Files changed

• server/src/app/api/search/route.ts — added integration header after Exa client init
• server/src/app/api/content/route.ts — added integration header after Exa client init

Code example

const searchClient = new Exa(SEARCH_API_KEY);
(searchClient as any).headers.set('x-exa-integration', 'prismercloud');

The as any cast is needed because headers is marked private in the exa-js type definitions, but is a standard Headers instance at runtime.

Test plan

• Verify /api/search returns results as before
• Verify /api/content returns content as before
• Confirm x-exa-integration header is present in outgoing requests (visible in Exa dashboard or network logs)