fix(config): require PLATFORM_URL explicitly, no hardcoded domain default

The previous fix defaulted PLATFORM_URL to this deployment's own domain
directly in docker-compose.prod.yml — any other deployment of this repo
would silently inherit it if they forgot to set their own. Now there is
no default at all: the compose file requires the env var to be set, and
the backend refuses to start in production if PLATFORM_URL still equals
the dev value, mirroring the existing SECRET_KEY enforcement.

backend/app/config.py

@@ -211,3 +211,14 @@ if _is_production:
                 f"Set a strong value via the {name} environment variable "
                 f"before running in production."
             )
+
+    # PLATFORM_URL has no safe production default — it's baked into every
+    # emailed link (set-password, notifications). Falling back silently to
+    # the dev value (or to some other deployment's hardcoded domain) would
+    # ship broken/wrong links without anyone noticing.
+    if settings.PLATFORM_URL == "http://localhost:5173":
+        raise RuntimeError(
+            "CRITICAL: PLATFORM_URL is not configured. Set it to this "
+            "deployment's real public frontend URL via the PLATFORM_URL "
+            "environment variable before running in production."
+        )