feat(config): add APP_BASE_URL setting for outbound link construction

Adds app_base_url config (default http://localhost:8000) used for all
outbound URLs: invitation emails, billing checkout redirects, signup
login links, portal return URLs.

Replaces hardcoded https://{main_domain} and localhost:8000 patterns.
Configurable per environment via APP_BASE_URL env var:
- Dev: http://localhost:8000 (or http://acme.localhost:9999)
- Prod: https://wizard.lu

main_domain is preserved for subdomain resolution and cookie config.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/billing/services/signup_service.py

@@ -617,7 +617,7 @@ class SignupService:
 
             # Build login URL
             login_url = (
-                f"https://{settings.main_domain}"
+                f"{settings.app_base_url.rstrip('/')}"
                 f"/store/{store.store_code}/dashboard"
             )