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/admin/routes.py

@@ -11,10 +11,10 @@ from quart import (
     request,
     jsonify
 )
-from suma_browser.app.redis_cacher import clear_all_cache
-from suma_browser.app.authz import require_admin
-from suma_browser.app.utils.htmx import is_htmx_request
-from config import config
+from shared.browser.app.redis_cacher import clear_all_cache
+from shared.browser.app.authz import require_admin
+from shared.browser.app.utils.htmx import is_htmx_request
+from shared.config import config
 from datetime import datetime
 
 def register(url_prefix):