docs: add DB table creation rules and guidelines

- Introduced essential rules for creating database tables, emphasizing the use of `VARCHAR(500)` for all business columns and the inclusion of five mandatory columns.
- Specified the requirement for registering three metadata tables: `table_labels`, `table_type_columns`, and `column_labels`.
- Outlined prohibited practices, including the direct use of certain database types and the necessity of including the `ON CONFLICT` clause for handling duplicates.

Made-with: Cursor

backend-node/src/services/multiTableExcelService.ts

@@ -972,7 +972,7 @@ class MultiTableExcelService {
          c.column_name,
          c.is_nullable AS db_is_nullable,
          c.column_default,
-         COALESCE(ttc.column_label, cl.column_label) AS column_label,
+         COALESCE(NULLIF(ttc.column_label, c.column_name), cl.column_label) AS column_label,
          COALESCE(ttc.reference_table, cl.reference_table) AS reference_table,
          COALESCE(ttc.is_nullable, cl.is_nullable) AS ttc_is_nullable
        FROM information_schema.columns c