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>

.env.example

@@ -72,6 +72,11 @@ LOG_FILE=logs/app.log
 # Your main platform domain
 MAIN_DOMAIN=wizard.lu
 
+# Full base URL for outbound links (emails, billing redirects, etc.)
+# Must include protocol and port if non-standard
+# Examples: http://localhost:8000, http://acme.localhost:9999, https://wizard.lu
+APP_BASE_URL=http://localhost:8000
+
 # Custom domain features
 # Enable/disable custom domains
 ALLOW_CUSTOM_DOMAINS=True