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/realism/llm/__init__.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """LLM backend for the realism library.
 
 Pluggable per the provider-subpackages convention (mirrors

decnet/realism/llm/base.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Backend protocol shared by every LLM transport.
 
 Deliberately narrow: realism consumers need one async ``generate``

decnet/realism/llm/circuit.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Process-local circuit breaker for LLM calls.
 
 Per-call timeouts (``asyncio.wait_for(llm.generate, timeout=...)``)

decnet/realism/llm/config.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """DB-backed LLM provider configuration for the realism subsystem.
 
 The module holds a process-level cached backend that callers obtain via

decnet/realism/llm/factory.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Backend dispatch.
 
 Reads ``DECNET_REALISM_LLM`` to pick a concrete :class:`LLMBackend`.

decnet/realism/llm/impl/__init__.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Concrete LLM-backend implementations.
 
 Importers go through :func:`decnet.realism.llm.get_llm`, not these

decnet/realism/llm/impl/fake.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """In-process fake backend for tests.
 
 Returns a canned string so the driver path can be exercised without an

decnet/realism/llm/impl/ollama.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Ollama backend — subprocess (local) or HTTP (remote).
 
 **Subprocess mode** (default, ``base_url=None``)