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/tenancy/services/store_team_service.py

@@ -981,19 +981,10 @@ class StoreTeamService:
     ):
         """Send team invitation email."""
         from app.core.config import settings as app_settings
-        from app.core.environment import is_production
         from app.modules.messaging.services.email_service import EmailService
 
-        # Build acceptance URL
-        # Prod: https://{subdomain}.{main_domain}/invitation/accept?token=...
-        # Dev:  http://localhost:8000/store/{store_code}/invitation/accept?token=...
-        main_domain = app_settings.main_domain.rstrip("/")
-        if is_production():
-            base_url = f"https://{store.subdomain}.{main_domain}"
-        else:
-            base_url = f"http://localhost:8000/store/{store.store_code}"
-
-        acceptance_link = f"{base_url}/invitation/accept?token={token}"
+        base_url = app_settings.app_base_url.rstrip("/")
+        acceptance_link = f"{base_url}/store/{store.store_code}/invitation/accept?token={token}"
 
         email_service = EmailService(db)
         email_service.send_template(