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.

tests/geoip/__init__.py

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

tests/geoip/conftest.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Per-package fixtures — flip DECNET_GEOIP_ENABLED back on for geoip tests
 and point the provider at a tmp dir so no real /var/lib/decnet paths get
 touched and no real RIR URL gets fetched.

tests/geoip/test_lookup.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Lookup index tests."""
 from __future__ import annotations
 

tests/geoip/test_parse.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Parser tests for RIR delegated-stats files."""
 from __future__ import annotations
 

tests/geoip/test_profiler_integration.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """_build_record must thread country fields through to the upsert payload."""
 from __future__ import annotations
 

tests/geoip/test_provider.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """RirProvider + factory + public API tests."""
 from __future__ import annotations
 

tests/geoip/test_ptr.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Unit tests for decnet.geoip.ptr — reverse-DNS resolver."""
 from __future__ import annotations