refactor(alpha): trim stale module surfaces

[onmax]

Summary

• remove stale convex/deprecated configuration references from docs and devtools metadata
• remove non-essential runtime metadata field databaseSource
• deduplicate useUserSignIn/useUserSignUp action-handle internals into a shared helper
• simplify internal session loading flow without changing public server utils API

Tests

• pnpm lint
• pnpm typecheck
• pnpm test
• pnpm build:docs

Related

• fix(nuxthub): use generated schema for drizzle adapter #153
• feat(schema): expose #auth/schema for stable table imports #154
• fix(nuxthub): use generated schema #156
• refactor(types): remove explicit any #158

[pkg-pr-new]

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npm i https://pkg.pr.new/nuxt-modules/better-auth/@onmax/nuxt-better-auth@159

commit: 41e1b80