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/campaigns/__init__.py

@@ -0,0 +1 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later

decnet/web/router/campaigns/api_events.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """SSE stream of campaign events — one connection per viewer.
 
 Subscribes to ``campaign.>`` on the bus for the duration of the

decnet/web/router/campaigns/api_get_campaign_detail.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """GET /api/v1/campaigns/{uuid} — single campaign row.
 
 Soft-merge handling: if the requested UUID has merged_into_uuid set,

decnet/web/router/campaigns/api_list_campaign_identities.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """GET /api/v1/campaigns/{uuid}/identities — identities for a campaign.
 
 Returns the ``AttackerIdentity`` rows whose ``campaign_id`` FK points

decnet/web/router/campaigns/api_list_campaigns.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """GET /api/v1/campaigns — paginated list of campaigns.
 
 Mirror of :mod:`decnet.web.router.identities.api_list_identities` for