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

Phase 1-3 of decoupling:
- path_setup.py adds project root to sys.path
- Blog-owned models in blog/models/ (ghost_content, snippet, tag_group)
- Re-export shims for shared models (user, kv, magic_link, menu_item)
- All imports updated: shared.infrastructure, shared.db, shared.browser, etc.
- No more cross-app post_id FKs in calendar/market/page_config

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)