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

src/main.lua

@@ -285,7 +285,7 @@ server:add_route("GET", "/health", function(request, server)
         timestamp = os.time(),
         source = version_info.source,
         features = {
-            smtp_configured = config.mail and config.mail.username ~= nil,
+            smtp_configured = config.smtp_default and config.smtp_default.host ~= nil,
             auth_enabled = true,
             rate_limiting = true,
             merkwerk_integrated = version_info.source == "merkwerk"