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/services/calendars.py

@@ -86,7 +86,9 @@ async def create_calendar(sess: AsyncSession, post_id: int, name: str) -> Calend
     name = (name or "").strip()
     if not name:
         raise CalendarError("Calendar name must not be empty.")
-    slug=slugify(name)
+    slug = slugify(name)
+    if slug in ("admin", "markets", "payments", "entries"):
+        raise CalendarError(f'"{slug}" is a reserved name and cannot be used as a calendar name.')
 
     # Ensure post exists (avoid silent FK errors in some DBs)
     raw = await fetch_data("blog", "post-by-id", params={"id": post_id}, required=False)