feat: make organization membership limit configurable via ORG_MEMBERSHIP_LIMIT env

[nizepart]

Summary

• Add ORG_MEMBERSHIP_LIMIT env variable to control the maximum members per organization
• Extract limit resolution into a testable helper (resolveOrgMembershipLimit)
• Preserve backward compatibility: when env is not set, better-auth's default of 100 remains

Closes #4639

Problem

The better-auth organization plugin enforces a membershipLimit of 100 by default. Self-hosted instances with larger teams cannot accept new invitations once this limit is reached, with no way to configure it without patching the source.

Changes

File	Change
packages/server/src/lib/membership-limit.ts	New helper: reads and validates ORG_MEMBERSHIP_LIMIT from env
packages/server/src/lib/auth.ts	Passes resolved limit to organization() plugin config
apps/dokploy/.env.example	Documents the new variable
apps/dokploy/__test__/organization/membership-limit.test.ts	Unit tests for the helper (9 tests)
apps/dokploy/__test__/organization/accept-invitation.test.ts	Integration tests for limit enforcement logic (12 tests)

Usage

# Set in .env to raise or effectively remove the limit
ORG_MEMBERSHIP_LIMIT=10000