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/v1/auth.py

@@ -2,9 +2,16 @@
 
 from fastapi import APIRouter, status
 
-from app.api.deps import AuthServiceDep, CurrentUser
-from app.api.schemas.auth import LoginRequest, RefreshRequest, TokenResponse
+from app.api.deps import AuthServiceDep, CurrentUser, UserServiceDep
+from app.api.schemas.auth import (
+    LoginRequest,
+    RefreshRequest,
+    RegisterRequest,
+    TokenResponse,
+)
 from app.api.schemas.user import UserResponse
+from app.core.config import get_settings
+from app.domain.errors import PermissionDeniedError
 from app.domain.tokens import TokenPair
 
 router = APIRouter(prefix="/auth", tags=["auth"])
@@ -23,6 +30,22 @@ async def login(body: LoginRequest, auth: AuthServiceDep) -> TokenResponse:
     return _to_token_response(pair)
 
 
+@router.post("/register", response_model=TokenResponse, status_code=status.HTTP_201_CREATED)
+async def register(
+    body: RegisterRequest, users: UserServiceDep, auth: AuthServiceDep
+) -> TokenResponse:
+    """Public self-service sign-up (gated by ``ALLOW_REGISTRATION``).
+
+    Registered accounts are always regular users — superusers are created
+    admin-only. On success the new account is logged straight in.
+    """
+    if not get_settings().allow_registration:
+        raise PermissionDeniedError("Registration is disabled on this instance.")
+    await users.create_user(username=body.username, password=body.password, is_superuser=False)
+    pair = await auth.login(body.username, body.password)
+    return _to_token_response(pair)
+
+
 @router.post("/refresh", response_model=TokenResponse)
 async def refresh(body: RefreshRequest, auth: AuthServiceDep) -> TokenResponse:
     pair = await auth.refresh(body.refresh_token)