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/bp/calendars/routes.py

@@ -21,7 +21,7 @@ from shared.browser.app.utils.htmx import is_htmx_request
 
 
 def register():
-    bp = Blueprint("calendars", __name__, url_prefix='/calendars')
+    bp = Blueprint("calendars", __name__, url_prefix='/admin')
     bp.register_blueprint(
       register_calendar(),
     )
@@ -32,7 +32,7 @@ def register():
 
     # ---------- Pages ----------
 
-    @bp.get("/calendars/")
+    @bp.get("/")
     @cache_page(tag="calendars")
     async def home(**kwargs):
         from shared.sexp.page import get_template_context
@@ -46,7 +46,7 @@ def register():
         return await make_response(html)
 
 
-    @bp.post("/calendars/new/")
+    @bp.post("/new/")
     @require_admin
     @clear_cache(tag="calendars", tag_scope="all")
     async def create_calendar(**kwargs):