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/coop_api.py

@@ -10,8 +10,8 @@ from quart import Blueprint, g, jsonify
 from sqlalchemy import select
 from sqlalchemy.orm import selectinload
 
-from models.menu_item import MenuItem
-from suma_browser.app.csrf import csrf_exempt
+from shared.models.menu_item import MenuItem
+from shared.browser.app.csrf import csrf_exempt
 
 
 def register() -> Blueprint:
@@ -53,7 +53,7 @@ def register() -> Blueprint:
         Return a Ghost post's key fields by slug.
         Called by market app for the landing page.
         """
-        from suma_browser.app.bp.blog.ghost_db import DBClient
+        from bp.blog.ghost_db import DBClient
 
         client = DBClient(g.s)
         posts = await client.posts_by_slug(slug, include_drafts=False)