Dissolve packages/db into apps/server

[hammadmajid]

Part of #3

Question

Dissolve packages/db into apps/server/src/db and bring the schema docs in line with better-auth's tables and the chosen storage model.

• Move the markdown next to the code it describes. Delete the package and its workspace entry.
• Rewrite auth.md and workspace.md around better-auth's user, session, organisation, member, and invitation tables, keeping the glossary's names in prose.
• Rewrite the conventions in the README for the storage model that won: which tables live in the shared D1, which live in a workspace object, and what that does to ULIDs, timestamps, and workspace_id columns.
• Drop every line that describes the code only sign in.
• Update the root README's package table.