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/prober/osfp/__init__.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Passive + active OS fingerprinting providers.
 
 Consumed by the profiler's `sniffer_rollup` (and, longer-term, by a

decnet/prober/osfp/base.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """OS-fingerprint provider protocol + OsMatch result shape.
 
 Each concrete provider (p0f v2 today; nmap-osdb / DECNET-observed DB

decnet/prober/osfp/factory.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """OS-fingerprint provider factory.
 
 Dispatch is env-driven (``DECNET_OSFP_PROVIDERS``, comma-separated),

decnet/prober/osfp/p0f/__init__.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """p0f v2 fingerprint database provider.
 
 Upstream: https://lcamtuf.coredump.cx/p0f.shtml  (p0f v2.0.8, 2006)

decnet/prober/osfp/p0f/format.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """p0f v2 ``.fp`` file parser.
 
 Format (from the DSL spec at the top of every shipped ``.fp`` file):

decnet/prober/osfp/p0f/provider.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """p0f v2 Provider — loads the vendored .fp databases and matches
 against observed TCP quirks.
 

decnet/prober/osfp/p0f/signature.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """p0f v2 signature + observation matching/scoring.
 
 A :class:`Signature` is one parsed row from a ``.fp`` file. A match