feat(organizations): config (enable/disable, autoCreate, domainMatching)

[PierreBrisorgueil]

Description

Add organization configuration to support both B2B (organizations visible) and B2C (organizations hidden) modes.

Details

Config section:

organizations: {
  enabled: true,        // false → B2C mode, organizations invisible to users
  autoCreate: true,     // auto-create a default organization on signup
  domainMatching: true, // match users to existing organizations by email domain
}

When enabled: false:

• Organization is auto-created at signup (invisible to user)
• No organization switcher, no organization pages in frontend
• User gets owner membership on their implicit organization
• All CASL abilities still work (scoped to implicit organization)
• Can switch to B2B later without refactoring

Config exposed via GET /api/auth/config response:

{
  "sign": { "in": true, "up": true },
  "organizations": { "enabled": true, "domainMatching": true }
}

Acceptance criteria

• Config toggles organization visibility
• B2C mode: organization auto-created silently at signup
• B2B mode: organization creation/joining flow at signup
• Config exposed in auth/config endpoint