chore: retarget develop cycle to 0.3.1

Only a bug fix (feed self link) landed since 0.3.0, so the next release is a
patch, not a minor. Correct the prematurely-opened 0.4.0 cycle.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "kill-the-news",
-  "version": "0.4.0-develop",
+  "version": "0.3.1-develop",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "kill-the-news",
-      "version": "0.4.0-develop",
+      "version": "0.3.1-develop",
       "license": "MIT",
       "dependencies": {
         "@hono/zod-openapi": "^1.4.0",

package.json

@@ -1,6 +1,6 @@
 {
   "name": "kill-the-news",
-  "version": "0.4.0-develop",
+  "version": "0.3.1-develop",
   "description": "Convert email newsletters into private RSS feeds using Cloudflare Workers",
   "main": "dist/worker.js",
   "scripts": {