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>

bp/menu_items/services/menu_items.py

@@ -2,7 +2,7 @@ from __future__ import annotations
 
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select, func
-from models.menu_item import MenuItem
+from shared.models.menu_item import MenuItem
 from models.ghost_content import Post