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>

Cargo.lock

@@ -1311,6 +1311,7 @@ dependencies = [
  "reqwest",
  "serde",
  "serde_json",
+ "serde_yaml_ng",
  "sqlx",
  "thiserror",
  "tracing",
@@ -1957,6 +1958,19 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "serde_yaml_ng"
+version = "0.10.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7b4db627b98b36d4203a7b458cf3573730f2bb591b28871d916dfa9efabfd41f"
+dependencies = [
+ "indexmap",
+ "itoa",
+ "ryu",
+ "serde",
+ "unsafe-libyaml",
+]
+
 [[package]]
 name = "sha1"
 version = "0.10.6"
@@ -2701,6 +2715,12 @@ version = "0.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b1d386ff53b415b7fe27b50bb44679e2cc4660272694b7b6f3326d8480823a94"
 
+[[package]]
+name = "unsafe-libyaml"
+version = "0.2.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "673aac59facbab8a9007c7f6108d11f63b603f7cabff99fabf650fea5c32b861"
+
 [[package]]
 name = "untrusted"
 version = "0.9.0"