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/bp/calendars/routes.py

@@ -32,7 +32,7 @@ def register():
 
     # ---------- Pages ----------
 
-    @bp.get("/")
+    @bp.get("/calendars/")
     @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("/new/")
+    @bp.post("/calendars/new/")
     @require_admin
     @clear_cache(tag="calendars", tag_scope="all")
     async def create_calendar(**kwargs):