fix: read store-github user/repo from env for Gitea

Hardcoded user: githubUsername (svemagie) and repo: "blog" were wrong
for Gitea where the org is giersig.eu and the repo is indiekit-blog.
Now reads from GITEA_CONTENT_USER / GITEA_CONTENT_REPO env vars with
sensible defaults.

Set in .env on server:
  GITEA_CONTENT_USER=giersig.eu
  GITEA_CONTENT_REPO=indiekit-blog

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

indiekit.config.mjs

@@ -320,8 +320,8 @@ export default {
     mountPath: "/micropub",
   },
   "@indiekit/store-github": {
-    user: githubUsername,
-    repo: "blog",
+    user: process.env.GITEA_CONTENT_USER || githubUsername,
+    repo: process.env.GITEA_CONTENT_REPO || "indiekit-blog",
     branch: "main",
     token: githubContentToken,
   },