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/bun/README.md

@@ -129,6 +129,9 @@ type User struct {
 - `default` - Default value
 - `rel` - Relationship definition
 - `type` - Explicit SQL type
+- `scanonly` - Excluded from INSERT/UPDATE: `GENERATED ... STORED` columns, and `GENERATED ALWAYS AS IDENTITY` columns that are not the primary key
+- `generated` - Marks a `GENERATED ... STORED` column
+- `identity` - Marks a `GENERATED ALWAYS AS IDENTITY` column (a primary key gets only this marker, since Bun drops `scanonly` fields from the PK list)
 
 ## Type Mapping