docs(stalwart): Update stability check status with investigation results

- Verified all infrastructure working correctly (SMTP/IMAP/LoadBalancer)
- Identified user SMTP issue as client configuration (port 143 vs 587)
- Documented proposed verification window restart (2026-07-13 to 2026-07-20)
- Original window had 2+ days with broken LoadBalancer (10.42.1.1 vs 178.105.17.239)

Related: DEV-300

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/stalwart/STABILITY-CHECK.md

@@ -59,11 +59,25 @@ This repository (`https://forgejo.basicstack.de/basicstack/stack.basicstack.de`)
 
 ---
 
-## Current Stability Check (DEV-235)
+## Current Stability Check (DEV-300)
 
-**Start**: 2026-07-11 11:04 UTC  
-**End**: 2026-07-18 11:04 UTC  
-**Status**: In progress (as of 2026-07-13)
+**Original Window (DEV-235)**: 2026-07-11 11:04 UTC → 2026-07-18 11:04 UTC  
+**Issue**: LoadBalancer used incorrect IP (10.42.1.1 instead of 178.105.17.239) until 2026-07-13 19:10 UTC  
+**Status**: Window restart pending board decision (as of 2026-07-13 20:37 UTC)
+
+**Proposed New Window**: 2026-07-13 20:37 UTC → 2026-07-20 20:37 UTC  
+**Rationale**: Clean 7-day verification with working external access
+
+### Investigation Results (2026-07-13)
+
+**Infrastructure Status**: ✅ ALL VERIFIED WORKING
+- SMTP (ports 25, 587, 465): Responding with correct ESMTP greeting
+- IMAP (ports 143, 993): Responding with correct IMAP greeting  
+- LoadBalancer external IP: 178.105.17.239 (correct)
+- Pod status: Running, 0 restarts
+- Monitoring: PrometheusRules and ServiceMonitors in place
+
+**User-Reported SMTP Issue**: Client configuration problem (Thunderbird connecting to port 143 instead of 587), not infrastructure issue
 
 ### Architecture Overview