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/snippet.py

@@ -0,0 +1,32 @@
+from __future__ import annotations
+
+from datetime import datetime
+
+from sqlalchemy import Integer, String, Text, DateTime, ForeignKey, UniqueConstraint, Index, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from shared.db.base import Base
+
+
+class Snippet(Base):
+    __tablename__ = "snippets"
+    __table_args__ = (
+        UniqueConstraint("user_id", "name", name="uq_snippets_user_name"),
+        Index("ix_snippets_visibility", "visibility"),
+    )
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    user_id: Mapped[int] = mapped_column(
+        ForeignKey("users.id", ondelete="CASCADE"), nullable=False,
+    )
+    name: Mapped[str] = mapped_column(String(255), nullable=False)
+    value: Mapped[str] = mapped_column(Text, nullable=False)
+    visibility: Mapped[str] = mapped_column(
+        String(20), nullable=False, default="private", server_default="private",
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now(),
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now(), onupdate=func.now(),
+    )