Colocate tests with their owners

[jasonhedman]

Summary

• move focused tests into tests folders beside their owning code
• use private _tests folders for App Router tests
• keep only repository-wide contracts and shared infrastructure in the root test folder
• update relative imports and database fixture paths after relocation

Verification

• pnpm check
• DATABASE_URL=postgresql://build:build@127.0.0.1:5432/build BETTER_AUTH_URL=http://localhost:3000 pnpm build