feat(pgsql): unique constraints on apply, matching unique index names, FK deferral

- migration writer now emits unique constraints (direct apply and live diff
  previously dropped them); idempotent via the existing DO block
- unique constraint adopts a same-named model unique index with
  ADD CONSTRAINT ... UNIQUE USING INDEX instead of building a second index
- dbml reader names unnamed unique indexes ukey_<table>_<cols> to match the
  unique constraint naming
- foreign key deferral: Constraint.Deferrable/InitiallyDeferred are *bool
  (nil = unspecified, never diffed); writers emit DEFERRABLE, DEFERRABLE
  INITIALLY DEFERRED and NOT DEFERRABLE; live reader reads condeferrable and
  condeferred; dbml reader/writer support deferrable and initially deferred
- tests and README/naming docs updated

pkg/writers/pgsql/writer.go

@@ -345,6 +345,7 @@ func (w *Writer) GenerateSchemaStatements(schema *models.Schema) ([]string, erro
 				TableName:      table.Name,
 				ConstraintName: constraint.Name,
 				Columns:        strings.Join(constraint.Columns, ", "),
+				UsingIndex:     adoptableUniqueIndex(table, constraint),
 			}
 
 			stmt, err := w.executor.ExecuteCreateUniqueConstraint(data)
@@ -411,7 +412,7 @@ func (w *Writer) GenerateSchemaStatements(schema *models.Schema) ([]string, erro
 				TargetColumns:  strings.Join(constraint.ReferencedColumns, ", "),
 				OnDelete:       onDelete,
 				OnUpdate:       onUpdate,
-				Deferrable:     false,
+				Deferral:       constraint.DeferralClause(),
 			}
 
 			stmt, err := w.executor.ExecuteCreateForeignKeyWithCheck(data)
@@ -1150,7 +1151,7 @@ func (w *Writer) writeIndexes(schema *models.Schema) error {
 			if indexName == "" {
 				indexType := "idx"
 				if index.Unique {
-					indexType = "uidx"
+					indexType = "ukey"
 				}
 				columnSuffix := strings.Join(index.Columns, "_")
 				indexName = fmt.Sprintf("%s_%s_%s", indexType, table.SQLName(), strings.ToLower(columnSuffix))
@@ -1236,6 +1237,7 @@ func (w *Writer) writeUniqueConstraints(schema *models.Schema) error {
 				TableName:      table.Name,
 				ConstraintName: constraint.Name,
 				Columns:        strings.Join(columnExprs, ", "),
+				UsingIndex:     adoptableUniqueIndex(table, constraint),
 			})
 			if err != nil {
 				return fmt.Errorf("failed to generate unique constraint: %w", err)
@@ -1380,7 +1382,7 @@ func (w *Writer) writeForeignKeys(schema *models.Schema) error {
 				TargetColumns:  strings.Join(targetColumns, ", "),
 				OnDelete:       onDelete,
 				OnUpdate:       onUpdate,
-				Deferrable:     true,
+				Deferral:       "DEFERRABLE",
 			}
 			sql, err := w.executor.ExecuteCreateForeignKeyWithCheck(data)
 			if err != nil {
@@ -1459,7 +1461,7 @@ func (w *Writer) writeForeignKeys(schema *models.Schema) error {
 				TargetColumns:  strings.Join(targetColumns, ", "),
 				OnDelete:       onDelete,
 				OnUpdate:       onUpdate,
-				Deferrable:     false,
+				Deferral:       constraint.DeferralClause(),
 			}
 			sql, err := w.executor.ExecuteCreateForeignKeyWithCheck(data)
 			if err != nil {