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_mgmt/__init__.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Swarm management endpoints for the React dashboard.
 
 These are *not* the unauthenticated /swarm routes mounted on the separate

decnet/web/router/swarm_mgmt/api_decommission_host.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """DELETE /swarm/hosts/{uuid} — decommission a worker from the dashboard.
 
 Also instructs the worker agent to stop all DECNET services and delete

decnet/web/router/swarm_mgmt/api_enroll_bundle.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Agent-enrollment bundles — the Wazuh-style one-liner flow.
 
 Three endpoints:

decnet/web/router/swarm_mgmt/api_list_deckies.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """GET /swarm/deckies — admin-gated list of decky shards across the fleet."""
 from __future__ import annotations
 

decnet/web/router/swarm_mgmt/api_list_hosts.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """GET /swarm/hosts — admin-gated list of enrolled workers for the dashboard.
 
 Fans out an ``AgentClient.health()`` probe to each host on every call and

decnet/web/router/swarm_mgmt/api_teardown_host.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """POST /swarm/hosts/{uuid}/teardown — remote teardown on a swarm worker.
 
 Body: ``{"decky_id": "..."}`` (optional). When ``decky_id`` is null/omitted