feat(slice-4): SmtpMailer — real mail over a configured relay, log-hygienic (INV-8, §6.6)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/platform/config.py

@@ -36,5 +36,35 @@ def cookie_secure() -> bool:
 
 
 def mailer_kind() -> str:
-    """Which mailer adapter to build: 'log' (dev/tests) or 'smtp' (deployed; SLICE-4)."""
+    """Which mailer adapter to build: 'log' (dev/tests) or 'smtp' (deployed)."""
     return os.environ.get("ECOMM_MAILER") or "log"
+
+
+# SMTP relay coordinates (deployed envs only; INV-8 — host/port/user/from are non-secret
+# overlay values, the password is a Secret Manager reference resolved by the deploy).
+
+
+def smtp_host() -> str:
+    return os.environ.get("ECOMM_SMTP_HOST", "")
+
+
+def smtp_port() -> int:
+    return int(os.environ.get("ECOMM_SMTP_PORT") or "587")
+
+
+def smtp_user() -> str:
+    return os.environ.get("ECOMM_SMTP_USER", "")
+
+
+def smtp_password() -> str:
+    return os.environ.get("ECOMM_SMTP_PASSWORD", "")
+
+
+def smtp_from() -> str:
+    """The From header; defaults to the relay user."""
+    return os.environ.get("ECOMM_SMTP_FROM") or smtp_user()
+
+
+def smtp_starttls() -> bool:
+    """STARTTLS on the relay connection (default on; disable only for odd relays)."""
+    return os.environ.get("ECOMM_SMTP_STARTTLS", "1").strip().lower() not in {"0", "false", "no", "off"}