chore: relicense to AGPL-3.0-or-later and add SPDX headers

Replaces LICENSE (GPLv3 -> AGPLv3) and prepends
`SPDX-License-Identifier: AGPL-3.0-or-later` to every source file
across decnet/, decnet_web/, tests/, scripts/, and tools/.

Rationale: closes the GPLv3 ASP loophole so any party operating a
modified DECNET as a network service must offer their modified
source. Personal copyright (Samuel Paschuan) + inbound=outbound
contributions make a future unilateral relicense infeasible.

- LICENSE: full AGPL-3.0 text (gnu.org/licenses/agpl-3.0.txt)
- COPYRIGHT: project copyright notice
- tools/add_spdx_headers.py: idempotent header injector
  (shebang- and PEP 263-aware)

Touches 1565 source files (.py, .ts, .tsx, .js, .jsx, .css, .sh).
No behavior change; comments only.

decnet/templates/ssh/_build_stealth.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python3
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """
 Build-time helper: merge capture Python sources, XOR+gzip+base64 pack them
 and the capture.sh loop, and render the final /entrypoint.sh from its

decnet/templates/ssh/capture.sh

@@ -1,4 +1,5 @@
 #!/bin/bash
+# SPDX-License-Identifier: AGPL-3.0-or-later
 # systemd-journal relay helper: mirrors newly-written files under a
 # monitored set of paths into the coredump staging directory and emits
 # a structured journal line per event.

decnet/templates/ssh/emit_capture.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python3
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """
 Emit an RFC 5424 `file_captured` line to stdout.
 

decnet/templates/ssh/entrypoint.sh

@@ -1,4 +1,5 @@
 #!/bin/bash
+# SPDX-License-Identifier: AGPL-3.0-or-later
 set -e
 
 # Configure root password (default: admin)

decnet/templates/ssh/instance_seed.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python3
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """
 Per-instance stealth seeding for honeypot service templates.
 

decnet/templates/ssh/syslog_bridge.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python3
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """
 Shared RFC 5424 syslog helper used by service containers.