feat(blog): add markdown blog sourced from a gitea repo

posts are markdown files with yaml frontmatter (title, slug, date;
optional draft/public) in the grenade/blog repo. the worker's new
BlogSource polls the repo — one branch-tip request when nothing
changed — and upserts posts into events with source='blog' and
occurred_at from the frontmatter date, so imported posts keep their
original publish dates and backfill the contribution graph.

- new /v1/blog and /v1/blog/{slug} endpoints over the existing
  EventReader port; drafts stay hidden via the public gate
- new /blog and /blog/:slug routes, nav link, activity-feed entry
  with post icon and filter toggle; relative image srcs resolve to
  gitea raw urls
- shared Markdown component extracted from ProjectPage
- vite proxy target overridable via API_PROXY_TARGET for local dev

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

ui/src/App.css

@@ -178,6 +178,19 @@ a.hot-pink {
   max-width: 100%;
 }
 
+.blog-post img {
+  max-width: 100%;
+  height: auto;
+  border-radius: 6px;
+}
+
+.blog-post pre {
+  background: rgba(0, 0, 0, 0.2);
+  padding: 0.75rem;
+  border-radius: 4px;
+  overflow-x: auto;
+}
+
 .site-footer {
   margin-top: 3rem;
   padding: 1rem 0;