docs: document db-init-job for automatic postgresql setup in litellm and langfuse

[pavlochaikivskyi]

Summary

Documents the new db-init-job Helm hook that automatically creates PostgreSQL databases and users for
LiteLLM and Langfuse during deployment, replacing the previous manual SQL setup step.

Changes

• Removed standalone PostgreSQL Setup step from LiteLLM workflow; moved manual SQL to <details> spoiler
  in manual deployment guide
• Added dbInitJob configuration section to LiteLLM configure-values.md and Langfuse deployment
  prerequisites
• Added codemie-postgresql admin secret instructions to both LiteLLM and Langfuse manual/automated
  deployment guides
• Updated deploy-langfuse.sh automated deployment docs to reflect new PostgreSQL credential prompts
• Renumbered LiteLLM deployment steps (1→4) and removed postgres-setup from sidebar

Testing

• Tested locally with npm start
• All pages render correctly
• Images display properly
• Internal links work
• Sidebar navigation works

Quality Checks

• npm run check passes (typecheck + lint + commitlint)
• No MDX compilation errors
• No raw angle brackets (<text> must be `<text>`)
• Sidebar references document IDs (not filenames)
• Images stored locally next to content (not in static/img/)
• Commit messages follow Conventional Commits
• No secrets or credentials in documentation

Additional Notes

[github-actions]

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