feat: decouple events from shared_lib, add app-owned models

Phase 1-3 of decoupling:
- path_setup.py adds project root to sys.path
- Events-owned models in events/models/ (calendars with all related models)
- All imports updated: shared.infrastructure, shared.db, shared.browser, etc.
- Calendar uses container_type/container_id instead of post_id FK
- CalendarEntryPost uses content_type/content_id (generic content refs)

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

events_api.py

@@ -11,7 +11,7 @@ from sqlalchemy import select, update, func
 from sqlalchemy.orm import selectinload
 
 from models.calendars import CalendarEntry, Calendar, Ticket
-from suma_browser.app.csrf import csrf_exempt
+from shared.browser.app.csrf import csrf_exempt
 
 
 def register() -> Blueprint: