Add a Blogger delivery channel with one-time OAuth setup

BloggerDeliverer posts each briefing (markdown -> HTML, H1 becomes the
post title with the report headline) via the Blogger API v3 using a
long-lived refresh token; `us2kr blogger-auth` runs the loopback OAuth
flow once and prints the BLOGGER_* lines for .env. Enabled only when
all four BLOGGER_* settings are present.

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

pyproject.toml

@@ -15,6 +15,7 @@ dependencies = [
     "python-dotenv>=1.0",
     "httpx>=0.27",
     "requests>=2.31",
+    "markdown>=3.6",
 ]
 
 [project.optional-dependencies]