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/gorm/README.md

@@ -203,6 +203,14 @@ The writer automatically generates relationship fields:
 - Relationship field names use 3-letter prefixes
 - Includes proper `gorm` tags with `foreignKey` and `references`
 
+## Read-only Columns
+
+| Column | Tags |
+|---|---|
+| `GENERATED ... STORED` | `<-:false;generated` |
+| `GENERATED ALWAYS AS IDENTITY`, not PK | `<-:false;identity` |
+| `GENERATED ALWAYS AS IDENTITY`, PK | `identity` only |
+
 ## Notes
 
 - Model names are prefixed with "Model" (e.g., `ModelUser`)