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/migration_writer.go

@@ -222,6 +222,13 @@ func (w *MigrationWriter) generateSchemaScripts(model, current *models.Schema) (
 	}
 	scripts = append(scripts, indexScripts...)
 
+	// Phase 4.5: Create unique constraints (Priority 185)
+	uniqueScripts, err := w.generateUniqueConstraintScripts(model, current)
+	if err != nil {
+		return nil, fmt.Errorf("failed to generate unique constraint scripts: %w", err)
+	}
+	scripts = append(scripts, uniqueScripts...)
+
 	// Phase 5: Create foreign keys (Priority 195)
 	fkScripts, err := w.generateForeignKeyScripts(model, current, droppedFKs)
 	if err != nil {
@@ -751,6 +758,92 @@ func buildIndexColumnExpressionsFiltered(table *models.Table, index *models.Inde
 	return columnExprs
 }
 
+// generateUniqueConstraintScripts generates ADD CONSTRAINT UNIQUE scripts for model unique
+// constraints that are missing from, or differ in, the live schema.
+func (w *MigrationWriter) generateUniqueConstraintScripts(model, current *models.Schema) ([]MigrationScript, error) {
+	scripts := make([]MigrationScript, 0)
+
+	currentTables := make(map[string]*models.Table)
+	if current != nil {
+		for _, table := range current.Tables {
+			currentTables[strings.ToLower(table.Name)] = table
+		}
+	}
+
+	for _, modelTable := range model.Tables {
+		currentTable := currentTables[strings.ToLower(modelTable.Name)]
+
+		for _, constraint := range sortConstraints(modelTable.Constraints) {
+			if constraint.Type != models.UniqueConstraint {
+				continue
+			}
+
+			if currentTable != nil {
+				if currentConstraint, exists := lookupConstraint(currentTable.Constraints, constraint.Name); exists && constraintsEqual(constraint, currentConstraint) {
+					continue
+				}
+			}
+
+			columnExprs := make([]string, 0, len(constraint.Columns))
+			for _, colName := range constraint.Columns {
+				if col, ok := modelTable.Columns[colName]; ok {
+					columnExprs = append(columnExprs, col.SQLName())
+				}
+			}
+			if len(columnExprs) == 0 {
+				continue
+			}
+
+			sql, err := w.executor.ExecuteCreateUniqueConstraint(CreateUniqueConstraintData{
+				SchemaName:     model.Name,
+				TableName:      modelTable.Name,
+				ConstraintName: constraint.Name,
+				Columns:        strings.Join(columnExprs, ", "),
+				UsingIndex:     adoptableUniqueIndex(modelTable, constraint),
+			})
+			if err != nil {
+				return nil, err
+			}
+
+			scripts = append(scripts, MigrationScript{
+				ObjectName: fmt.Sprintf("%s.%s.%s", model.Name, modelTable.Name, constraint.Name),
+				ObjectType: "create unique constraint",
+				Schema:     model.Name,
+				Priority:   185,
+				Sequence:   len(scripts),
+				Body:       sql,
+			})
+		}
+	}
+
+	return scripts, nil
+}
+
+// adoptableUniqueIndex returns the name of the model's unique index that the given unique
+// constraint can be built on with ADD CONSTRAINT ... UNIQUE USING INDEX, so the constraint does
+// not create a second index (or collide with the index's name). The index must carry the
+// constraint's name, be a plain non-partial btree index and cover exactly the constraint's
+// columns. Index creation runs before constraint creation, so this holds whether or not the
+// live database already has the index. Returns "" when none qualifies.
+func adoptableUniqueIndex(table *models.Table, constraint *models.Constraint) string {
+	index, ok := table.Indexes[constraint.Name]
+	if !ok || !index.Unique || index.Where != "" || len(index.Include) > 0 {
+		return ""
+	}
+	if index.Type != "" && !strings.EqualFold(index.Type, "btree") {
+		return ""
+	}
+	if len(index.Columns) != len(constraint.Columns) {
+		return ""
+	}
+	for i, col := range index.Columns {
+		if !strings.EqualFold(col, constraint.Columns[i]) {
+			return ""
+		}
+	}
+	return index.Name
+}
+
 // generateForeignKeyScripts generates ADD CONSTRAINT FOREIGN KEY scripts using templates.
 // forceRecreate is a set of FK constraint keys (schema.table.name) that must be recreated
 // even if unchanged, because their referenced PK was dropped and recreated.
@@ -800,6 +893,12 @@ func (w *MigrationWriter) generateForeignKeyScripts(model, current *models.Schem
 					onUpdate = strings.ToUpper(constraint.OnUpdate)
 				}
 
+				// An unspecified deferral keeps the historical default of DEFERRABLE.
+				deferral := constraint.DeferralClause()
+				if deferral == "" {
+					deferral = "DEFERRABLE"
+				}
+
 				sql, err := w.executor.ExecuteCreateForeignKey(CreateForeignKeyData{
 					SchemaName:     model.Name,
 					TableName:      modelTable.Name,
@@ -810,6 +909,7 @@ func (w *MigrationWriter) generateForeignKeyScripts(model, current *models.Schem
 					TargetColumns:  strings.Join(constraint.ReferencedColumns, ", "),
 					OnDelete:       onDelete,
 					OnUpdate:       onUpdate,
+					Deferral:       deferral,
 				})
 				if err != nil {
 					return nil, err
@@ -1149,6 +1249,9 @@ func constraintsEqual(c1, c2 *models.Constraint) bool {
 		if !fkActionsEqual(c1.OnDelete, c2.OnDelete) || !fkActionsEqual(c1.OnUpdate, c2.OnUpdate) {
 			return false
 		}
+		if !models.DeferralMatches(c1, c2) {
+			return false
+		}
 	}
 
 	return true