refactor: rename platform_domain → main_domain to avoid confusion with platform.domain

The setting `settings.platform_domain` (the global/main domain like "wizard.lu")
was easily confused with `platform.domain` (per-platform domain like "rewardflow.lu").
Renamed to `settings.main_domain` / `MAIN_DOMAIN` env var across the entire codebase.

Also updated docs to reflect the refactored store detection logic with
`is_platform_domain` / `is_subdomain_of_platform` guards.

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

scripts/seed/install.py

@@ -339,7 +339,7 @@ def validate_configuration(env_vars: dict) -> dict:
     # -------------------------------------------------------------------------
     # Platform Domain
     # -------------------------------------------------------------------------
-    domain = env_vars.get("PLATFORM_DOMAIN", "orion.lu")
+    domain = env_vars.get("MAIN_DOMAIN", "orion.lu")
     if domain != "orion.lu":
         results["domain"] = {
             "status": "ok",
@@ -350,7 +350,7 @@ def validate_configuration(env_vars: dict) -> dict:
         results["domain"] = {
             "status": "warning",
             "message": "Using default domain",
-            "items": ["Set PLATFORM_DOMAIN for your deployment"]
+            "items": ["Set MAIN_DOMAIN for your deployment"]
         }
 
     # -------------------------------------------------------------------------