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/core/config.py

@@ -98,6 +98,11 @@ class Settings(BaseSettings):
     # =============================================================================
     main_domain: str = "wizard.lu"
 
+    # Full base URL for outbound links (emails, redirects, etc.)
+    # Must include protocol and port if non-standard.
+    # Examples: http://localhost:8000, http://acme.localhost:9999, https://wizard.lu
+    app_base_url: str = "http://localhost:8000"
+
     # Custom domain features
     allow_custom_domains: bool = True
     require_domain_verification: bool = True