Rename coop to blog throughout codebase

- coop_url() → blog_url(), AUTH_APP default → "blog"
- Session cookie: coop_session → blog_session
- Config keys: coop_root/coop_title → market_root/market_title
- All Jinja templates: coop_url → blog_url, coop_title → market_title
- Template blocks: coop-child-header → blog-child-header

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

db/session.py

@@ -7,7 +7,7 @@ from quart import Quart, g
 DATABASE_URL = (
     os.getenv("DATABASE_URL_ASYNC")
     or os.getenv("DATABASE_URL")
-    or "postgresql+asyncpg://localhost/coop"
+    or "postgresql+asyncpg://localhost/blog"
 )
 
 _engine = create_async_engine(