Complete rename from thrivesight-core → digital-brain-backend

[tchjhnsn]

Repo transferred from tchjhnsn to soulpolity and renamed 2026-04-19. Remaining in-repo cleanup:

1. Update package.json name field to digital-brain-backend (or @soulpolity/digital-brain-backend if scoped)
2. Search & replace any ThriveSight-branded identifiers in code: imports, class names, env vars, doc strings
3. Rewrite README.md reflecting Polity AI Lab naming and the sibling architecture (reflect-graph-contract, signal-address, federated-graph)
4. Update internal CLAUDE.md if present

Context: this code was extracted from the ThriveSight hackathon. The rewriting was done on the vault-side architecture docs already (see projects/polity/platforms/ai-lab/product/architecture/). Bring the code into alignment.