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/routes/api/store_addons.py

@@ -144,7 +144,7 @@ def purchase_addon(
     store = billing_service.get_store(db, store_id)
 
     # Build URLs
-    base_url = f"https://{settings.main_domain}"
+    base_url = settings.app_base_url.rstrip("/")
     success_url = f"{base_url}/store/{store.store_code}/billing?addon_success=true"
     cancel_url = f"{base_url}/store/{store.store_code}/billing?addon_cancelled=true"
 

app/modules/billing/routes/api/store_checkout.py

@@ -59,7 +59,7 @@ def create_checkout_session(
 
     store_code = subscription_service.get_store_code(db, store_id)
 
-    base_url = f"https://{settings.main_domain}"
+    base_url = settings.app_base_url.rstrip("/")
     success_url = f"{base_url}/store/{store_code}/billing?success=true"
     cancel_url = f"{base_url}/store/{store_code}/billing?cancelled=true"
 
@@ -87,7 +87,7 @@ def create_portal_session(
     merchant_id, platform_id = subscription_service.resolve_store_to_merchant(db, store_id, current_user.token_platform_id)
 
     store_code = subscription_service.get_store_code(db, store_id)
-    return_url = f"https://{settings.main_domain}/store/{store_code}/billing"
+    return_url = f"{settings.app_base_url.rstrip('/')}/store/{store_code}/billing"
 
     result = billing_service.create_portal_session(db, merchant_id, platform_id, return_url)