feat: mark generated and identity columns across writers and DBML

- bun: scanonly+generated for GENERATED columns; scanonly+identity for ALWAYS identity (PK gets identity only)
- gorm: <-:false+generated / identity, same primary key rule
- dbml: write GENERATED/IDENTITY column notes and read them back into the model
- pgsql: emit generation/identity clauses in migration create-table and add-column, no DEFAULT
- readmes updated

pkg/writers/pgsql/templates/create_table.tmpl

@@ -2,7 +2,9 @@ CREATE TABLE IF NOT EXISTS {{qual_table .SchemaName .TableName}} (
 {{- range $i, $col := .Columns}}
 {{- if $i}},{{end}}
   {{quote_ident $col.Name}} {{$col.Type}}
-{{- if $col.Default}} DEFAULT {{$col.Default}}{{end}}
+{{- if $col.Generated}} GENERATED ALWAYS AS ({{$col.Generated}}) STORED{{end}}
+{{- if $col.Identity}} GENERATED {{$col.Identity}} AS IDENTITY{{end}}
+{{- if and $col.Default (not $col.Generated) (not $col.Identity)}} DEFAULT {{$col.Default}}{{end}}
 {{- if $col.NotNull}} NOT NULL{{end}}
 {{- end}}
 );