fix: Add SELF_HOSTING env variable to disable image optimization

[kaigritun]

Fixes #1467

Problem: Images were broken on self-hosted deployments because Next.js image optimization requires Vercel infrastructure.

Solution: Added SELF_HOSTING environment variable that disables Next.js image optimization when set to true.

Changes:

• Added SELF_HOSTING env variable to env schema
• Updated next.config.ts to set images.unoptimized based on SELF_HOSTING
• Added SELF_HOSTING to .env.example with documentation
• Updated SELF_HOSTING.md to include new required variable

Testing: Set SELF_HOSTING=true in apps/app/.env and verify images load correctly on risk and vendors pages.

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