docs: prefer AGENT_CONTROL_DB_URL in configuration docs

[lan17]

Summary

• update docs examples to use AGENT_CONTROL_DB_URL for server database configuration
• document the AGENT_CONTROL_DB_* variables in the configuration reference
• update server component snippets and Docker examples to match the current env var names

Validation

• pnpm exec mintlify validate

Notes

• The repo build script currently points to mintlify build, which no longer exists in the installed Mintlify CLI. I used pnpm exec mintlify validate instead.

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