feat(db): set up NeonDB + Drizzle ORM with membership schema

[cdcore09]

Summary

Set up the database infrastructure for the membership management system. Design and implement the schema, configure NeonDB serverless Postgres, and establish the Drizzle ORM migration workflow.

Requirements

• Configure NeonDB project and connection (serverless driver)
• Set up Drizzle ORM with type-safe schema definitions
• Design schema tables: users, profiles, memberships, roles, sessions, audit_log
• Profile fields: name, pronouns, bio, institution, title, photo_url, github, linkedin, orcid, website, research_interests (array), working_groups, affinity_groups
• Role system: member, admin (board/staff), super_admin
• Audit log table for admin actions (who did what, when)
• Set up Drizzle Kit for migration generation and management
• Seed script for development data
• Document schema in a migrations README

Context

Parent issue: #1916
This is the foundation — must be completed before auth, profiles, or admin dashboard.

Implementation Notes

• Use drizzle-orm/neon-http for serverless edge compatibility
• Schema should use Postgres-native types (jsonb for flexible fields, text arrays for tags)
• Consider Drizzle relations for type-safe joins
• All timestamps should be timestamp with time zone