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",