Phase 0+1: native post writes, Ghost no longer write-primary

- Final sync script with HTML verification + author→user migration
- Make ghost_id nullable on posts/authors/tags, add UUID/timestamp defaults
- Add user profile fields (bio, slug, profile_image, etc.) to User model
- New PostUser M2M table (replaces post_authors for new posts)
- PostWriter service: direct DB CRUD with Lexical rendering, optimistic
  locking, AP federation, tag upsert
- Rewrite create/edit/settings routes to use PostWriter (no Ghost API calls)
- Neuter Ghost webhooks (post/page/author/tag → 204 no-op)
- Disable Ghost startup sync

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/models/ghost_content.py

@@ -19,7 +19,7 @@ class Tag(Base):
     __tablename__ = "tags"
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
-    ghost_id: Mapped[str] = mapped_column(String(64), index=True, unique=True, nullable=False)
+    ghost_id: Mapped[Optional[str]] = mapped_column(String(64), index=True, unique=True, nullable=True)
 
     slug: Mapped[str] = mapped_column(String(191), index=True, nullable=False)
     name: Mapped[str] = mapped_column(String(255), nullable=False)
@@ -50,8 +50,8 @@ class Post(Base):
     __tablename__ = "posts"
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
-    ghost_id: Mapped[str] = mapped_column(String(64), index=True, unique=True, nullable=False)
-    uuid: Mapped[str] = mapped_column(String(64), unique=True, nullable=False)
+    ghost_id: Mapped[Optional[str]] = mapped_column(String(64), index=True, unique=True, nullable=True)
+    uuid: Mapped[str] = mapped_column(String(64), unique=True, nullable=False, server_default=func.gen_random_uuid())
     slug: Mapped[str] = mapped_column(String(191), index=True, nullable=False)
 
     title: Mapped[str] = mapped_column(String(500), nullable=False)
@@ -89,8 +89,8 @@ class Post(Base):
     comment_id: Mapped[Optional[str]] = mapped_column(String(191))
 
     published_at: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True))
-    updated_at: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True))
-    created_at: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True))
+    updated_at: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True), server_default=func.now())
+    created_at: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True), server_default=func.now())
     deleted_at: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True))
 
     user_id: Mapped[Optional[int]] = mapped_column(
@@ -136,7 +136,7 @@ class Author(Base):
     __tablename__ = "authors"
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
-    ghost_id: Mapped[str] = mapped_column(String(64), index=True, unique=True, nullable=False)
+    ghost_id: Mapped[Optional[str]] = mapped_column(String(64), index=True, unique=True, nullable=True)
 
     slug: Mapped[str] = mapped_column(String(191), index=True, nullable=False)
     name: Mapped[str] = mapped_column(String(255), nullable=False)
@@ -192,3 +192,17 @@ class PostTag(Base):
     sort_order: Mapped[int] = mapped_column(Integer, default=0, nullable=False)
 
 
+class PostUser(Base):
+    """Multi-author M2M: links posts to users (cross-DB, no FK on user_id)."""
+    __tablename__ = "post_users"
+
+    post_id: Mapped[int] = mapped_column(
+        ForeignKey("posts.id", ondelete="CASCADE"),
+        primary_key=True,
+    )
+    user_id: Mapped[int] = mapped_column(
+        Integer, primary_key=True, index=True,
+    )
+    sort_order: Mapped[int] = mapped_column(Integer, default=0, nullable=False)
+
+