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/web/router/swarm_updates/__init__.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Remote Updates — master dashboard's surface for pushing code to workers.
 
 These are *not* the swarm-controller's /swarm routes (those run on a

decnet/web/router/swarm_updates/api_list_host_releases.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """GET /swarm-updates/hosts — per-host updater health + release slots.
 
 Fans out an ``UpdaterClient.health()`` probe to every enrolled host that

decnet/web/router/swarm_updates/api_push_update.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """POST /swarm-updates/push — fan a tarball of the master's tree to workers.
 
 Mirrors the ``decnet swarm update`` CLI flow: build the tarball once,

decnet/web/router/swarm_updates/api_push_update_self.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """POST /swarm-updates/push-self — push only to workers' /update-self.
 
 Use case: the agent is fine but the updater itself needs an upgrade (e.g.

decnet/web/router/swarm_updates/api_rollback_host.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """POST /swarm-updates/rollback — manual rollback on a single host.
 
 Calls the worker updater's ``/rollback`` which swaps the ``current``