Move calendar management to /{slug}/admin/ and reserve slug

- Change calendars blueprint prefix from /calendars to /admin
- Simplify routes from /calendars/ to / within blueprint
- Reserve admin, markets, payments, entries as calendar slugs
- Update blog admin nav link to /{slug}/admin/

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

events/app.py

@@ -90,8 +90,8 @@ def create_app() -> "Quart":
         url_prefix="/<slug>",
     )
 
-    # Calendars nested under post slug: /<slug>/<calendar_slug>/...
-    # Listing stays at /<slug>/calendars/, individual at /<slug>/<calendar_slug>/
+    # Calendar admin under post slug: /<slug>/admin/
+    # Individual calendars at /<slug>/<calendar_slug>/
     app.register_blueprint(
         register_calendars(),
         url_prefix="/<slug>",