fix(host): resolve Host-005..011 — async startup, HOCON escaping, port-conflict check, dead-config cleanup, migration retry, log-level wiring; Host-002 flagged

src/ScadaLink.Host/StartupRetry.cs

@@ -0,0 +1,48 @@
+namespace ScadaLink.Host;
+
+/// <summary>
+/// Bounded retry-with-backoff for startup preconditions.
+///
+/// Host-010 / REQ-HOST-4a: a Central node applies/validates database migrations
+/// before the host begins serving traffic. In container orchestration the database
+/// and the app frequently start together, so the database may be briefly
+/// unreachable. Rather than crashing the process on the first connection failure,
+/// the migration step is wrapped in this bounded exponential backoff: it tolerates a
+/// short outage and only fails fatally once attempts are exhausted.
+/// </summary>
+public static class StartupRetry
+{
+    public static async Task ExecuteWithRetryAsync(
+        string operationName,
+        Func<Task> operation,
+        int maxAttempts,
+        TimeSpan initialDelay,
+        ILogger logger,
+        CancellationToken cancellationToken = default)
+    {
+        var delay = initialDelay;
+        for (var attempt = 1; ; attempt++)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            try
+            {
+                await operation();
+                if (attempt > 1)
+                    logger.LogInformation(
+                        "Startup operation '{Operation}' succeeded on attempt {Attempt}.",
+                        operationName, attempt);
+                return;
+            }
+            catch (Exception ex) when (attempt < maxAttempts)
+            {
+                logger.LogWarning(ex,
+                    "Startup operation '{Operation}' failed on attempt {Attempt}/{MaxAttempts}; " +
+                    "retrying in {Delay}.",
+                    operationName, attempt, maxAttempts, delay);
+                await Task.Delay(delay, cancellationToken);
+                // Exponential backoff, capped so the total wait stays bounded.
+                delay = TimeSpan.FromTicks(Math.Min(delay.Ticks * 2, TimeSpan.FromSeconds(30).Ticks));
+            }
+        }
+    }
+}