Register the @better-auth/passkey plugin alongside the existing
better-auth setup. Adds a passkey table to the Drizzle schema with FK
cascade to user, RLS enabled, and a userId index. The rpID is derived
dynamically from CLIENT_URL so Portless dev (e.g.
https://wt44.localhost) and production both work without extra env
config. The plugin is placed before customSession to preserve the
requirement that customSession remain last.
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