feat: SMTP mail stack and separate server.env deploy

Add mail.php with STARTTLS sending, test scripts, server.env split from
local .env, admin@ postbox docs, DKIM DNS support-ticket notes, and todo 11
for boka relay.

Co-authored-by: Cursor <cursoragent@cursor.com>

config/bootstrap.php

@@ -3,8 +3,8 @@
 declare(strict_types=1);
 
 /**
- * Load configuration from private/.env (outside document root).
- * Called from httpdocs/*.php via require on private/config/bootstrap.php.
+ * Load configuration from private/server.env (outside document root).
+ * Local .env is never deployed — see server.env.example.
  */
 
 function bootstrap(): array
@@ -15,12 +15,29 @@ function bootstrap(): array
         return $config;
     }
 
-    $envFile = dirname(__DIR__) . '/.env';
-    $config = loadEnvFile($envFile);
+    $config = loadEnvFile(envFilePath());
 
     return $config;
 }
 
+function envFilePath(): string
+{
+    $baseDir = dirname(__DIR__);
+
+    // Webspace: private/config/bootstrap.php → private/server.env
+    if (basename($baseDir) === 'private') {
+        return $baseDir . '/server.env';
+    }
+
+    // Local repo: .env on this machine (never deployed)
+    $local = $baseDir . '/.env';
+    if (is_readable($local)) {
+        return $local;
+    }
+
+    return $baseDir . '/server.env';
+}
+
 function env(string $key, ?string $default = null): ?string
 {
     $value = bootstrap()[$key] ?? null;