Sign up, sign in, and invite through better-auth

[hammadmajid]

Part of #3

Question

Bring better-auth into apps/server so sign up, sign in, and invite work end to end, with organisations as workspaces.

• Install better-auth with the Drizzle adapter on D1 and the organisation plugin. Every sign in method that needs no password, as the research ticket found them.
• Sign up creates the user, the workspace, and the owner membership. Invite creates a membership from a link.
• Sessions per the glossary: thirty days, sliding, sign out everywhere.
• Waitlist match at sign up: an email on the waitlist is marked as early bird, so the Polar discount can be attached later.
• Polar plugin present but inert when its secrets are absent, which is what a self hosted instance is.
• Mount it at /auth/* on the Worker, and fill rpc/base.ts's authed middleware so procedures can require a session; integration tests inside the Vitest pool against real D1.
• Record the better-auth decision and the self hosted instance definition as ADRs once it runs.