fix(config): lua 5.1 compatibility and multi-tenant validation

- Replace goto statements with if-not pattern for Lua 5.1 compatibility
- Validate mail config only for API keys with mail:send permissions
- Safe display of API key info for monitoring keys without mail config
- Fix health check SMTP detection for new config structure
- Multi-tenant system tested and working on port 7811

Fixes multi-tenant config parsing, validation, and health checks.
Related to DAW/furt#89

config/server.lua

@@ -67,11 +67,28 @@ print("  Default SMTP: " .. (config.smtp_default.host or "not configured"))
 local api_key_count = 0
 for key_name, key_config in pairs(config.api_keys) do
     api_key_count = api_key_count + 1
+
+    -- Check if this API key has mail permissions
+    local has_mail_permission = false
+    if key_config.permissions then
+        for _, perm in ipairs(key_config.permissions) do
+            if perm == "mail:send" or perm == "*" then
+                has_mail_permission = true
+                break
+            end
+        end
+    end
+
     local smtp_info = ""
     if key_config.mail_smtp_host then
         smtp_info = " (custom SMTP: " .. key_config.mail_smtp_host .. ")"
     end
-    print("  API Key: " .. key_config.name .. " -> " .. key_config.mail_to .. smtp_info)
+
+    if has_mail_permission then
+        print("  API Key: " .. key_config.name .. " -> " .. key_config.mail_to .. smtp_info)
+    else
+        print("  API Key: " .. key_config.name .. " (no mail)" .. smtp_info)
+    end
 end
 print("  Total API Keys: " .. api_key_count)