feat(auth): public self-service registration (ALLOW_REGISTRATION)

Add POST /auth/register: creates a non-superuser then auto-logs in,
returning the same TokenResponse as login. Gated by the new
allow_registration setting (env ALLOW_REGISTRATION, default true);
when disabled it raises PermissionDeniedError (403). Accounts remain
admin-only for superusers.

Tests cover create+login, duplicate (409), short password (422), and
the disabled (403) path.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/api/schemas/auth.py

@@ -10,6 +10,11 @@ class LoginRequest(BaseModel):
     password: str = Field(min_length=1)
 
 
+class RegisterRequest(BaseModel):
+    username: str = Field(min_length=1, max_length=64)
+    password: str = Field(min_length=8)
+
+
 class RefreshRequest(BaseModel):
     refresh_token: str