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/db/sqlmodel_repo/topology/__init__.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """MazeNET topology repository methods.
 
 The full domain spans ~700 lines of methods across topologies, LANs,

decnet/web/db/sqlmodel_repo/topology/_core.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Topology table CRUD + the optimistic-locking helpers that the
 sibling LAN / decky / edge / mutation mixins call through MRO."""
 from __future__ import annotations

decnet/web/db/sqlmodel_repo/topology/deckies.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Topology decky CRUD + the running-decky listing the fleet aggregator
 calls through MRO."""
 from __future__ import annotations

decnet/web/db/sqlmodel_repo/topology/edges.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Topology edge CRUD + status-event log."""
 from __future__ import annotations
 

decnet/web/db/sqlmodel_repo/topology/lans.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """LAN CRUD within a topology."""
 from __future__ import annotations
 

decnet/web/db/sqlmodel_repo/topology/mutations.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Live-reconciler mutation queue: enqueue + atomic claim + state writes."""
 from __future__ import annotations