License the project under AGPL-3.0-or-later

Adds the verbatim license text as LICENSE, sets license metadata in
both package manifests, and links it from both README.md files.
Chosen specifically for the network-copyleft clause (AGPL §13): a
modified version run as a hosted service must offer its source to
that service's users, which plain GPL's distribution-only trigger
doesn't cover.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/pyproject.toml

@@ -2,6 +2,7 @@
 name = "ds-chat"
 version = "0.1.0"
 description = "DS Chat backend service"
+license = { text = "AGPL-3.0-or-later" }
 requires-python = ">=3.11"
 dependencies = [
     "fastapi>=0.115",