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/__init__.py

@@ -0,0 +1,14 @@
+from .ghost_content import Post, Author, Tag, PostAuthor, PostTag, PostLike
+from .snippet import Snippet
+from .tag_group import TagGroup, TagGroupTag
+
+# Shared models — canonical definitions live in shared/models/
+from shared.models.ghost_membership_entities import (
+    GhostLabel, UserLabel,
+    GhostNewsletter, UserNewsletter,
+    GhostTier, GhostSubscription,
+)
+from shared.models.menu_item import MenuItem
+from shared.models.kv import KV
+from shared.models.magic_link import MagicLink
+from shared.models.user import User