Bump to v2026.9.3

Switches to a date-based version scheme (YYYY.M.D, with .2/.3/etc.
appended for additional same-day releases) instead of semver.

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

backend/pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "ds-chat"
-version = "1.1.0"
+version = "2026.9.3"
 description = "DS Chat backend service"
 license = { text = "AGPL-3.0-or-later" }
 requires-python = ">=3.11"

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "frontend",
   "private": true,
-  "version": "1.1.0",
+  "version": "2026.9.3",
   "license": "AGPL-3.0-or-later",
   "type": "module",
   "scripts": {