fix(scripts): add fallback logic for OpenSMTPD binary detection

The scripts were hardcoded to look for OpenSMTPD at a custom workspace
path, but the Dockerfile installs it via apt to /usr/sbin/smtpd. This
change adds fallback logic to check multiple locations:

1. Custom workspace path (for backward compatibility)
2. System path /usr/sbin/smtpd
3. Anywhere in PATH

Also adds graceful handling when OpenSMTPD is not installed, logging
an informative message instead of failing with "No such file or directory".

scripts/healthcheck.sh

@@ -174,8 +174,20 @@ main() {
 
     # Check OpenSMTPD service (ports 25, 587, 465)
     health_log "INFO" "=== OpenSMTPD Service ==="
-    OPENSMTPD_BINARY="/workspace/src/backend/app/opensmtpd/install/sbin/smtpd"
-    if [ -x "$OPENSMTPD_BINARY" ]; then
+    OPENSMTPD_CUSTOM_BINARY="/workspace/src/backend/app/opensmtpd/install/sbin/smtpd"
+    OPENSMTPD_SYSTEM_BINARY="/usr/sbin/smtpd"
+    
+    # Determine which binary to use (prefer custom, fallback to system)
+    OPENSMTPD_BINARY=""
+    if [ -x "$OPENSMTPD_CUSTOM_BINARY" ]; then
+        OPENSMTPD_BINARY="$OPENSMTPD_CUSTOM_BINARY"
+    elif [ -x "$OPENSMTPD_SYSTEM_BINARY" ]; then
+        OPENSMTPD_BINARY="$OPENSMTPD_SYSTEM_BINARY"
+    elif command -v smtpd &>/dev/null; then
+        OPENSMTPD_BINARY="$(command -v smtpd)"
+    fi
+    
+    if [ -n "$OPENSMTPD_BINARY" ]; then
         # Check SMTP port 25
         if check_port 25 "SMTP"; then
             health_log "INFO" "✓ SMTP port 25 is listening"
@@ -205,7 +217,7 @@ main() {
             health_log "WARN" "⚠ OpenSMTPD process not found (may be disabled)"
         fi
     else
-        health_log "INFO" "OpenSMTPD not installed at expected location, skipping SMTP checks"
+        health_log "INFO" "OpenSMTPD not installed (checked: $OPENSMTPD_CUSTOM_BINARY, $OPENSMTPD_SYSTEM_BINARY, PATH), skipping SMTP checks"
     fi
 
     # ttyd service has been removed, no longer checking port 4501