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>

crates/moments-core/src/presentation.rs

@@ -5,6 +5,7 @@
 
 use moments_entities::{Event, Source, TimelineItem};
 
+pub mod blog;
 mod bugzilla;
 mod gitea;
 mod github;
@@ -16,5 +17,6 @@ pub fn reshape(event: &Event) -> TimelineItem {
         Source::Gitea => gitea::reshape(event),
         Source::Hg => hg::reshape(event),
         Source::Bugzilla => bugzilla::reshape(event),
+        Source::Blog => blog::reshape(event),
     }
 }