fix: add ResolutionFlow service account to own default tree steps in library

Default/system trees had no author_id (NULL), causing a NOT NULL violation
when syncing steps to step_library.created_by on publish.

- Add is_service_account flag to users table (migration 4f4137ce)
- Add service_account.py: idempotent ensure_service_account() creates
  noreply@resolutionflow.com with unusable password on startup
- Cache service account ID on app.state at lifespan startup
- Add get_service_account_id() FastAPI dep (returns None in tests)
- sync_steps_from_tree: resolve author_id or service_account_id as created_by
- create_tree: set author_id=service_account_id for is_default trees
- Migration 1490781700bc: backfill author_id on 31 existing default trees

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

backend/app/core/step_sync.py

@@ -109,14 +109,22 @@ async def sync_steps_from_tree(
     tree_id: UUID,
     tree_type: str,
     tree_structure: dict,
-    author_id: UUID,
+    author_id: Optional[UUID],
     account_id: Optional[UUID],
     is_public: bool,
+    service_account_id: Optional[UUID] = None,
 ) -> int:
     """Upsert step library entries from a published tree.
 
     Returns the number of steps synced.
+
+    For default/system trees that have no author_id, pass service_account_id
+    so that created_by is set to the ResolutionFlow service account.
     """
+    resolved_author_id = author_id or service_account_id
+    if not resolved_author_id:
+        return 0
+
     now = datetime.now(timezone.utc)
     extracted = list(extract_steps_for_sync(tree_structure, tree_type))
 
@@ -157,7 +165,7 @@ async def sync_steps_from_tree(
                 "title": step_data["title"],
                 "step_type": step_data["step_type"],
                 "content": json.dumps(step_data["content"]),
-                "created_by": str(author_id) if author_id else None,
+                "created_by": str(resolved_author_id),
                 "account_id": str(account_id) if account_id else None,
                 "visibility": visibility,
                 "source_tree_id": str(tree_id),