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/readers/pgsql/queries.go

@@ -439,7 +439,9 @@ func (r *Reader) queryForeignKeys(schemaName string) (map[string][]*models.Const
 			fcl.relname AS referenced_table_name,
 			fatt.attname AS referenced_column,
 			` + actionCase("con.confupdtype") + ` AS update_rule,
-			` + actionCase("con.confdeltype") + ` AS delete_rule
+			` + actionCase("con.confdeltype") + ` AS delete_rule,
+			con.condeferrable AS deferrable,
+			con.condeferred AS initially_deferred
 		FROM pg_catalog.pg_constraint con
 		JOIN pg_catalog.pg_class cl ON cl.oid = con.conrelid
 		JOIN pg_catalog.pg_namespace ns ON ns.oid = cl.relnamespace
@@ -470,6 +472,8 @@ func (r *Reader) queryForeignKeys(schemaName string) (map[string][]*models.Const
 		referencedColumns []string
 		updateRule        string
 		deleteRule        string
+		deferrable        bool
+		initiallyDeferred bool
 	}
 
 	fkMap := make(map[string]*fkData)
@@ -479,8 +483,9 @@ func (r *Reader) queryForeignKeys(schemaName string) (map[string][]*models.Const
 		var foreignSchema, foreignTable, foreignColumn string
 		var referencedSchema, referencedTable, referencedColumn string
 		var updateRule, deleteRule string
+		var deferrable, initiallyDeferred bool
 
-		if err := rows.Scan(&schema, &tableName, &constraintName, &foreignSchema, &foreignTable, &foreignColumn, &referencedSchema, &referencedTable, &referencedColumn, &updateRule, &deleteRule); err != nil {
+		if err := rows.Scan(&schema, &tableName, &constraintName, &foreignSchema, &foreignTable, &foreignColumn, &referencedSchema, &referencedTable, &referencedColumn, &updateRule, &deleteRule, &deferrable, &initiallyDeferred); err != nil {
 			return nil, err
 		}
 
@@ -497,6 +502,8 @@ func (r *Reader) queryForeignKeys(schemaName string) (map[string][]*models.Const
 				referencedColumns: []string{},
 				updateRule:        updateRule,
 				deleteRule:        deleteRule,
+				deferrable:        deferrable,
+				initiallyDeferred: initiallyDeferred,
 			}
 		}
 
@@ -517,6 +524,9 @@ func (r *Reader) queryForeignKeys(schemaName string) (map[string][]*models.Const
 		constraint.ReferencedColumns = fk.referencedColumns
 		constraint.OnUpdate = fk.updateRule
 		constraint.OnDelete = fk.deleteRule
+		deferrable, initiallyDeferred := fk.deferrable, fk.initiallyDeferred
+		constraint.Deferrable = &deferrable
+		constraint.InitiallyDeferred = &initiallyDeferred
 
 		tableKey := fk.schema + "." + fk.tableName
 		foreignKeys[tableKey] = append(foreignKeys[tableKey], constraint)