Add SPDX headers to every Go file

GPLv3's "How to Apply These Terms" asks for a notice in each source file;
this is the modern two-line SPDX form of it rather than the full paragraph.
82 files, including tests.

The blank line after the header is load-bearing. In Go a comment block
immediately preceding `package X` becomes the package doc comment, so
without the separator the SPDX lines would be absorbed into the doc for the
eleven packages whose doc.go (or main.go) opens with one, and `go doc` would
print them. Verified it doesn't.

internal/recovery/apply.go

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package recovery
 
 import (

internal/recovery/apply_test.go

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package recovery
 
 import (

internal/recovery/doc.go

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 // Package recovery implements supervision of the Stalwart recovery-mode process and the settings apply step.
 // See ARCHITECTURE.md §4.4 for the design.
 package recovery

internal/recovery/exectest_test.go

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package recovery
 
 import (

internal/recovery/health.go

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package recovery
 
 import (

internal/recovery/main_test.go

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package recovery
 
 import (

internal/recovery/process.go

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package recovery
 
 import (

internal/recovery/process_test.go

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package recovery
 
 import (

internal/recovery/recovery.go

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package recovery
 
 import (

internal/recovery/recovery_test.go

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package recovery
 
 import (

internal/recovery/report.go

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package recovery
 
 import (