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>

bp/calendar/admin/routes.py

@@ -5,8 +5,8 @@ from quart import (
 )
 
 
-from suma_browser.app.authz import require_admin
-from suma_browser.app.redis_cacher import clear_cache
+from shared.browser.app.authz import require_admin
+from shared.browser.app.redis_cacher import clear_cache
 
 
 
@@ -17,7 +17,7 @@ def register():
     @bp.get("/")
     @require_admin
     async def admin(calendar_slug: str, **kwargs):
-        from suma_browser.app.utils.htmx import is_htmx_request
+        from shared.browser.app.utils.htmx import is_htmx_request
 
         # Determine which template to use based on request type
         if not is_htmx_request():