feat: enforce email as primary user identity in SaaS mode

All users in SaaS mode must have an email address. The bootstrap creates
the admin user with primaryEmail set to SAAS_ADMIN_EMAIL (defaults to
<SAAS_ADMIN_USER>@<PUBLIC_HOST>). This prevents the admin from being
locked out when self-service registration (which requires email) is
enabled via the Email Connector UI.

Documentation updated across all CLAUDE.md files, .env.example,
user-manual.md, and installer submodule (README, .env.example, compose).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main/java/net/siegeln/cameleer/saas/config/CLAUDE.md

@@ -1,5 +1,9 @@
 # Auth & Security Config
 
+## User identity
+
+**Email is the primary user identity** in SaaS mode. All users must have an email address — Logto enforces this via `signUp.identifiers: ["email"]` when registration is enabled. The bootstrap creates the admin user with `primaryEmail` set to `SAAS_ADMIN_EMAIL` (defaults to `<SAAS_ADMIN_USER>@<PUBLIC_HOST>`). Self-service registration requires email verification via a configured email connector (vendor UI at `/vendor/email`).
+
 ## Auth enforcement
 
 - All API endpoints enforce OAuth2 scopes via `@PreAuthorize("hasAuthority('SCOPE_xxx')")` annotations