feat(auth): invite links for several people, invite by email, registration with approval or email domains

- Invites: link /?invite=CODE, copy link, send by email, max uses with counter
- Registration modes closed | approval | domain | domain+approval | open, waiting list with approve/reject
- REGISTRATION_MODE / REGISTRATION_DOMAINS lock the admin setting; ALLOW_REGISTRATION=true is honoured

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "mindsift-frontend",
-  "version": "0.22.2",
+  "version": "0.23.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "mindsift-frontend",
-      "version": "0.22.2",
+      "version": "0.23.0",
       "license": "Unlicense",
       "dependencies": {
         "@fontsource-variable/inter": "5.3.0",