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/dbml/reader_test.go

@@ -3,6 +3,7 @@ package dbml
 import (
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"git.warky.dev/wdevs/relspecgo/pkg/models"
@@ -842,9 +843,9 @@ func TestConstraintNaming(t *testing.T) {
 		t.Logf("Available constraints: %v", getKeys(postsTable.Constraints))
 	}
 
-	// Test unique index naming: uidx_table_columns
-	if _, exists := postsTable.Indexes["uidx_posts_slug"]; !exists {
-		t.Error("Expected unique index 'uidx_posts_slug' not found")
+	// Test unique index naming: ukey_table_columns, matching the unique constraint name
+	if _, exists := postsTable.Indexes["ukey_posts_slug"]; !exists {
+		t.Error("Expected unique index 'ukey_posts_slug' not found")
 		t.Logf("Available indexes: %v", getKeys(postsTable.Indexes))
 	}
 
@@ -1121,3 +1122,69 @@ func TestReader_DuplicateIndexNameInTableKept(t *testing.T) {
 		}
 	}
 }
+
+func TestReader_ForeignKeyDeferral(t *testing.T) {
+	dbml := `Table users {
+  id integer [pk]
+}
+
+Table posts {
+  id integer [pk]
+  user_id integer [ref: > users.id, deferrable, initially deferred]
+  editor_id integer
+  plain_id integer [ref: > users.id]
+}
+
+Table comments {
+  id integer [pk]
+  post_id integer
+  author_id integer
+}
+
+Ref: comments.post_id > posts.id [delete: cascade, deferrable]
+Ref: comments.author_id > users.id [deferrable, initially deferred]
+`
+	path := filepath.Join(t.TempDir(), "deferral.dbml")
+	if err := os.WriteFile(path, []byte(dbml), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	db, err := NewReader(&readers.ReaderOptions{FilePath: path}).ReadDatabase()
+	if err != nil {
+		t.Fatalf("ReadDatabase() error = %v", err)
+	}
+
+	find := func(table, column string) *models.Constraint {
+		for _, s := range db.Schemas {
+			for _, tb := range s.Tables {
+				if tb.Name != table {
+					continue
+				}
+				for _, c := range tb.Constraints {
+					if c.Type == models.ForeignKeyConstraint && len(c.Columns) == 1 && c.Columns[0] == column {
+						return c
+					}
+				}
+			}
+		}
+		t.Fatalf("FK %s.%s not found", table, column)
+		return nil
+	}
+
+	tests := []struct {
+		table, column string
+		want          string
+	}{
+		{"posts", "user_id", "DEFERRABLE INITIALLY DEFERRED"},
+		{"posts", "plain_id", ""},
+		{"comments", "post_id", "DEFERRABLE"},
+		{"comments", "author_id", "DEFERRABLE INITIALLY DEFERRED"},
+	}
+	for _, tt := range tests {
+		if got := find(tt.table, tt.column).DeferralClause(); got != tt.want {
+			t.Errorf("%s.%s deferral = %q, want %q", tt.table, tt.column, got, tt.want)
+		}
+	}
+	if got := find("comments", "post_id").OnDelete; !strings.EqualFold(got, "cascade") {
+		t.Errorf("delete action lost, got %q", got)
+	}
+}