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>

app.py

@@ -6,11 +6,11 @@ from quart import g, request
 from jinja2 import FileSystemLoader, ChoiceLoader
 from sqlalchemy import select
 
-from shared.factory import create_base_app
-from config import config
-from models import KV
+from shared.infrastructure.factory import create_base_app
+from shared.config import config
+from shared.models import KV
 
-from suma_browser.app.bp import (
+from bp import (
     register_auth_bp,
     register_blog_bp,
     register_admin,
@@ -27,9 +27,9 @@ async def coop_context() -> dict:
     - menu_items: direct DB query (coop owns this data)
     - cart_count/cart_total: fetched from cart internal API
     """
-    from shared.context import base_context
-    from suma_browser.app.bp.menu_items.services.menu_items import get_all_menu_items
-    from shared.internal_api import get as api_get
+    from shared.infrastructure.context import base_context
+    from bp.menu_items.services.menu_items import get_all_menu_items
+    from shared.infrastructure.internal_api import get as api_get
 
     ctx = await base_context()