Add User Blocking System with Smart Session Management

[Dharp02]

Implement comprehensive user blocking functionality with organization-level granularity and intelligent blocking logic that handles multi-org scenarios.

Backend Changes:

• Add Better Auth hooks to prevent blocked users from signing in (all auth methods)
• Add session.create hook to block users blocked from ALL their orgs
• Add user.create hook to prevent signup with blocked email addresses
• Auto-cleanup stale blocking records when user is re-added to org
• Auto-readd users as members when unblocking
• Add block/unblock endpoints with reason tracking
• Add migration for user.blocked field (array of org-level blocks)
• Add activity log events for blocking/unblocking

Frontend Changes:

• Add blockMessage to session context to expose blocking errors
• Display user-friendly blocking messages in LoginForm
• Add Block/Unblock UI in OrganizationMembersPage with reason dialog
• Handle blocking errors for both email and GitHub OAuth sign-in
• Prevent blocked emails from creating new accounts

Blocking Behavior:

• User blocked from SOME orgs → can sign in, blocked orgs invisible
• User blocked from ALL orgs → cannot sign in at all, clear error message
• Stale blocks (user re-added manually) → ignored and cleaned up automatically
• GitHub OAuth → blocked via same session hook as email auth
• Unblocking → automatically restores org membership

Tests:

• All 26 organization tests pass including enhanced unblock verification