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>

blog/sexp/sexp_components.py

@@ -148,7 +148,7 @@ def _post_admin_nav_html(ctx: dict) -> str:
     market_url_fn = ctx.get("market_url")
     if callable(events_url_fn):
         for url_fn, path, label in [
-            (events_url_fn, f"/{slug}/calendars/calendars/", "calendars"),
+            (events_url_fn, f"/{slug}/admin/", "calendars"),
             (market_url_fn, f"/{slug}/", "markets"),
             (events_url_fn, f"/{slug}/payments/", "payments"),
         ]: