Fix market and calendar URL routing

Market: blog links now use market_url('/{slug}/') instead of
events_url('/{slug}/markets/'), matching the market service's
actual route structure /<page_slug>/<market_slug>/.

Calendar: flatten route from /<slug>/calendars/<calendar_slug>/
to /<slug>/<calendar_slug>/ by changing the events app blueprint
prefix and moving listing routes to explicit /calendars/ paths.
Update all hardcoded calendar URL paths across blog and events
services (Python + Jinja templates).

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

events/app.py

@@ -90,10 +90,11 @@ def create_app() -> "Quart":
         url_prefix="/<slug>",
     )
 
-    # Calendars nested under post slug: /<slug>/calendars/...
+    # Calendars nested under post slug: /<slug>/<calendar_slug>/...
+    # Listing stays at /<slug>/calendars/, individual at /<slug>/<calendar_slug>/
     app.register_blueprint(
         register_calendars(),
-        url_prefix="/<slug>/calendars",
+        url_prefix="/<slug>",
     )
 
     # Markets nested under post slug: /<slug>/markets/...