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>

models/ghost_membership_entities.py

@@ -0,0 +1,12 @@
+# Re-export from canonical shared location
+from shared.models.ghost_membership_entities import (
+    GhostLabel, UserLabel,
+    GhostNewsletter, UserNewsletter,
+    GhostTier, GhostSubscription,
+)
+
+__all__ = [
+    "GhostLabel", "UserLabel",
+    "GhostNewsletter", "UserNewsletter",
+    "GhostTier", "GhostSubscription",
+]