feat(external users): email sanity + mock SMTP

- SMTP has a mock to enable end to end test and validate the whole path
     (via OXICLOUD_SMTP_MOCK)
    - add email normalisation ( including punicode)
    - api to share to external user

tests/common/server.env

@@ -22,3 +22,15 @@ RUST_LOG=warn
 # grow up limits for tests
 OXICLOUD_RATE_LIMIT_REFRESH_MAX=360
 OXICLOUD_RATE_LIMIT_LOGIN_MAX=360
+
+# Magic-link / external-users flow (PR 9). The mock SMTP captures every
+# outbound message in-process so external_users.hurl can retrieve the
+# invitation body and follow the magic-link URL. The `SMTP_FROM` value
+# is required so the mock can build a valid Message; host/port are
+# irrelevant in mock mode but kept set for completeness.
+OXICLOUD_SMTP_MOCK=true
+OXICLOUD_SMTP_HOST=localhost
+OXICLOUD_SMTP_PORT=25
+OXICLOUD_SMTP_FROM='OxiCloud Tests <test@oxicloud.local>'
+OXICLOUD_SMTP_TLS=none
+OXICLOUD_ALLOW_EXTERNAL_USERS=true