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/fragments.tmpl

@@ -3,7 +3,9 @@
 {{/* Column definition fragment */}}
 {{- define "column_definition" -}}
 {{.Name}} {{.Type}}
-{{- if .Default}} DEFAULT {{.Default}}{{end}}
+{{- if .Generated}} GENERATED ALWAYS AS ({{.Generated}}) STORED{{end}}
+{{- if .Identity}} GENERATED {{.Identity}} AS IDENTITY{{end}}
+{{- if and .Default (not .Generated) (not .Identity)}} DEFAULT {{.Default}}{{end}}
 {{- if .NotNull}} NOT NULL{{end}}
 {{- end -}}