refactor(core): finalize no-convex db dx

[onmax]

Summary

• remove remaining convex-specific core surface (options/docs/files)
• keep provider auto-detection (nuxthub or none) and support manual Better Auth database precedence
• harden runtime metadata to report effective provider (user when manual db is used) and keep databaseSource
• align tests/devtools/docs with the new semantics
• keep temporary @libsql/linux-x64-gnu dev dependency for contributor test stability, with follow-up note

Supersedes #106.

Tests

• pnpm test
• pnpm typecheck
• pnpm lint

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/nuxt-modules/better-auth/@onmax/nuxt-better-auth@113

commit: 864a8fc