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>

blog/alembic/env.py

@@ -13,6 +13,7 @@ MODELS = [
 
 TABLES = frozenset({
     "posts", "authors", "post_authors", "tags", "post_tags",
+    "post_users",
     "snippets", "tag_groups", "tag_group_tags",
     "menu_items", "menu_nodes", "kv",
     "page_configs",

blog/alembic/versions/0004_ghost_id_nullable_and_defaults.py

@@ -0,0 +1,67 @@
+"""Make ghost_id nullable, add defaults, create post_users M2M table.
+
+Revision ID: 0004
+Revises: 0003_add_page_configs
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0004"
+down_revision = "0003_add_page_configs"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # Make ghost_id nullable
+    op.alter_column("posts", "ghost_id", existing_type=sa.String(64), nullable=True)
+    op.alter_column("authors", "ghost_id", existing_type=sa.String(64), nullable=True)
+    op.alter_column("tags", "ghost_id", existing_type=sa.String(64), nullable=True)
+
+    # Add server defaults for Post
+    op.alter_column(
+        "posts", "uuid",
+        existing_type=sa.String(64),
+        server_default=sa.text("gen_random_uuid()"),
+    )
+    op.alter_column(
+        "posts", "updated_at",
+        existing_type=sa.DateTime(timezone=True),
+        server_default=sa.text("now()"),
+    )
+    op.alter_column(
+        "posts", "created_at",
+        existing_type=sa.DateTime(timezone=True),
+        server_default=sa.text("now()"),
+    )
+
+    # Create post_users M2M table (replaces post_authors for new posts)
+    op.create_table(
+        "post_users",
+        sa.Column("post_id", sa.Integer, sa.ForeignKey("posts.id", ondelete="CASCADE"), primary_key=True),
+        sa.Column("user_id", sa.Integer, primary_key=True),
+        sa.Column("sort_order", sa.Integer, nullable=False, server_default="0"),
+    )
+    op.create_index("ix_post_users_user_id", "post_users", ["user_id"])
+
+    # Backfill post_users from post_authors for posts that already have user_id.
+    # This maps each post's authors to the post's user_id (primary author).
+    # Multi-author mapping requires the full sync script.
+    op.execute("""
+        INSERT INTO post_users (post_id, user_id, sort_order)
+        SELECT p.id, p.user_id, 0
+        FROM posts p
+        WHERE p.user_id IS NOT NULL
+          AND p.deleted_at IS NULL
+        ON CONFLICT DO NOTHING
+    """)
+
+
+def downgrade():
+    op.drop_table("post_users")
+    op.alter_column("posts", "created_at", existing_type=sa.DateTime(timezone=True), server_default=None)
+    op.alter_column("posts", "updated_at", existing_type=sa.DateTime(timezone=True), server_default=None)
+    op.alter_column("posts", "uuid", existing_type=sa.String(64), server_default=None)
+    op.alter_column("tags", "ghost_id", existing_type=sa.String(64), nullable=False)
+    op.alter_column("authors", "ghost_id", existing_type=sa.String(64), nullable=False)
+    op.alter_column("posts", "ghost_id", existing_type=sa.String(64), nullable=False)