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/auth/services/auth_operations.py

@@ -10,8 +10,8 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import selectinload
 
-from models import User, MagicLink, UserNewsletter
-from config import config
+from shared.models import User, MagicLink, UserNewsletter
+from shared.config import config
 
 
 def get_app_host() -> str:

bp/auth/services/login_redirect.py

@@ -1,7 +1,7 @@
 from urllib.parse import urlparse
 from quart import session
 
-from shared.urls import coop_url
+from shared.infrastructure.urls import coop_url
 
 
 LOGIN_REDIRECT_SESSION_KEY = "login_redirect_to"