Fix critical bug: never recommend 0 for pm.max_children in batch analyzer

ROOT CAUSE:
The batch analyzer calls calculate_optimal_php_settings() which relies on
calculate_max_children_memory_based(). When no active PHP-FPM processes exist
(common in ondemand mode with sparse traffic), both functions returned 0.

IMPACT:
- Recommending pm.max_children: 0 (completely invalid, breaks PHP-FPM)
- Causes silent failures in optimization reports
- Especially problematic with ondemand PM mode + low traffic domains

FIXES:
1. calculate_max_children_memory_based():
   • When no processes detected: return 20 instead of 0
   • When invalid parameters: return 20 instead of 0

2. calculate_optimal_php_settings():
   • Added CRITICAL safety check: if final_max_children <= 0, use 20
   • Ensures output is always safe regardless of calculation errors

DEFAULTS:
- Memory-based: 20 (safe minimum when no process data available)
- Traffic-based: Uses actual peak concurrent if available
- Safety guardrail: 20 minimum in all code paths

This prevents invalid recommendations and ensures batch analyzer always
provides sensible, actionable optimization guidance.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

lib/php-calculator-improved.sh

@@ -84,7 +84,7 @@ calculate_max_children_memory_based() {
     local total_ram_mb="$2"
 
     if [ -z "$total_ram_mb" ] || [ -z "$username" ]; then
-        echo "0|Invalid parameters"
+        echo "20|Invalid parameters"
         return
     fi
 
@@ -93,7 +93,9 @@ calculate_max_children_memory_based() {
     avg_kb=$(get_fpm_memory_usage "$username" 2>/dev/null || echo "0")
 
     if [ "$avg_kb" -eq 0 ]; then
-        echo "0|No active PHP-FPM processes found"
+        # No active processes detected (ondemand mode, or low traffic)
+        # Use safe default: 20 processes with assumed 50MB per process
+        echo "20|No active processes, using safe default"
         return
     fi
 
@@ -369,6 +371,12 @@ calculate_optimal_php_settings() {
         reason_prefix="Combined (memory: $memory_based_max, traffic: $traffic_based_max)"
     fi
 
+    # CRITICAL: Ensure we never recommend 0 or invalid values
+    if [ -z "$final_max_children" ] || [ "$final_max_children" -le 0 ]; then
+        final_max_children="20"
+        reason_prefix="Safe default (calculation failed or returned invalid value)"
+    fi
+
     # Recommend pm mode
     local pm_result
     pm_result=$(recommend_pm_mode "$peak_concurrent" "$((peak_concurrent / 2))" "$stability_factor")