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/canary/instrumenters/__init__.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Built-in canary instrumenters (operator-uploaded artifact mutation).
 
 Lazy-imported by :func:`decnet.canary.factory.get_instrumenter`.

decnet/canary/instrumenters/docx.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """DOCX instrumenter — inject a remote image into the body.
 
 DOCX files are zip archives carrying ``word/document.xml`` (the body)

decnet/canary/instrumenters/html.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """HTML instrumenter — append a 1×1 tracking pixel.
 
 Stdlib-only.  We don't parse the HTML; we just inject the ``<img>``

decnet/canary/instrumenters/image.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Image instrumenter — requires :mod:`PIL` (optional dependency).
 
 For PNG/JPEG/GIF we append a tEXt/EXIF chunk carrying the slug so

decnet/canary/instrumenters/passthrough.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Passthrough instrumenter — bytes go to disk unchanged.
 
 Used as the dispatch fallback for content types we can't safely

decnet/canary/instrumenters/pdf.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """PDF instrumenter — requires :mod:`pikepdf` (optional dependency).
 
 PDF embedding is non-trivial: the cleanest place to put a callback

decnet/canary/instrumenters/plain.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Plain-text / config-file instrumenter.
 
 Two embedding strategies, picked in order:

decnet/canary/instrumenters/xlsx.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """XLSX instrumenter — embed an external-image link.
 
 XLSX is structurally identical to DOCX (Office Open XML zip).  The