Fix docs AI smart contracts path

[samsamtrum]

Summary

• resolve the smart contracts source path from populateDB.js instead of relying on the process working directory
• load contracts from the checked-out smart-contracts/contracts folder instead of root node_modules

Why

This addresses #732 by removing the docs AI DB population dependency on node_modules/@nilfoundation/smart-contracts/contracts. The route can now find the Solidity sources from the repository layout directly.

Testing

• git diff --check
• Verified the resolved path points to smart-contracts/contracts and exists locally

Note: pnpm install --frozen-lockfile is currently blocked by an overrides/lockfile config mismatch with pnpm 11, and pnpm --filter docs_ai_backend exec tsc --noEmit triggers workspace install that fails on malformed registry metadata for hardhat-foundry in rollup-bridge-contracts.

Fixes #732