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>

path_setup.py

@@ -1,7 +1,9 @@
 import sys
 import os
 
-# Add the shared library submodule to the Python path
-_shared = os.path.join(os.path.dirname(os.path.abspath(__file__)), "shared_lib")
-if _shared not in sys.path:
-    sys.path.insert(0, _shared)
+_app_dir = os.path.dirname(os.path.abspath(__file__))
+_project_root = os.path.dirname(_app_dir)
+
+for _p in (_project_root, _app_dir):
+    if _p not in sys.path:
+        sys.path.insert(0, _p)