fix(§6/auth): reconcile by email in provision_user so OTC→OAuth doesn't 500 (v0.54.1)

The Gitea-OAuth callback's `auth.provision_user` matched an existing `users` row
only by `gitea_id`. A human who signed in first via OTC owns an email-only row
(`gitea_id` NULL); a later Gitea-OAuth sign-in with the same email missed that
row and INSERTed a new one, colliding on the `idx_users_email` case-insensitive
unique index → IntegrityError → 500 callback.

Fix (mirror of `otc.provision_or_link_user`): when no `gitea_id` row exists,
reconcile by email and link the OAuth identity (gitea_id/gitea_login/profile)
onto the existing email row. Owner-zero (§6.1) bootstrap applied on link
(matching the fresh-insert path); `permission_state` preserved so linking never
changes admission status as a side effect.

One §9-surfaced framework fragility of two. backend 680 green (+3).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "rfc-app-frontend",
   "private": true,
-  "version": "0.54.0",
+  "version": "0.54.1",
   "type": "module",
   "scripts": {
     "dev": "vite",