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.go

@@ -848,6 +848,13 @@ func (r *Reader) parseColumn(line, tableName, schemaName string) (*models.Column
 		}
 	}
 
+	// "deferrable" / "initially deferred" column settings apply to the inline foreign key
+	if constraint != nil && constraint.Type == models.ForeignKeyConstraint && attrs != "" {
+		for _, attr := range splitColumnAttrs(attrs) {
+			applyDeferralSetting(constraint, attr)
+		}
+	}
+
 	// Parse inline comment
 	if inlineComment != "" {
 		column.Comment = inlineComment
@@ -1081,11 +1088,13 @@ func (r *Reader) parseIndex(line, tableName, schemaName string) *models.Index {
 
 	// Generate name if not provided
 	if index.Name == "" {
-		prefix := "idx"
+		// Unique indexes share the ukey_ naming of unique constraints, so an index and a
+		// constraint on the same columns get the same name and can be merged on write.
+		prefix, name := "idx", tableName
 		if index.Unique {
-			prefix = "uidx"
+			prefix, name = "ukey", strings.TrimLeft(tableName, "_")
 		}
-		index.Name = fmt.Sprintf("%s_%s_%s", prefix, tableName, strings.Join(columns, "_"))
+		index.Name = fmt.Sprintf("%s_%s_%s", prefix, name, strings.Join(columns, "_"))
 	}
 
 	return index
@@ -1182,6 +1191,8 @@ func (r *Reader) parseRef(refStr string) *models.Constraint {
 					constraint.OnDelete = strings.TrimSpace(strings.TrimPrefix(action, "ondelete:"))
 				} else if strings.HasPrefix(action, "onupdate:") {
 					constraint.OnUpdate = strings.TrimSpace(strings.TrimPrefix(action, "onupdate:"))
+				} else {
+					applyDeferralSetting(constraint, action)
 				}
 			}
 		}
@@ -1190,6 +1201,18 @@ func (r *Reader) parseRef(refStr string) *models.Constraint {
 	return constraint
 }
 
+// applyDeferralSetting applies a "deferrable" or "initially deferred" Ref setting to a
+// foreign key constraint. Other settings are ignored.
+func applyDeferralSetting(constraint *models.Constraint, setting string) {
+	yes := true
+	switch strings.Join(strings.Fields(strings.ToLower(setting)), " ") {
+	case "deferrable":
+		constraint.Deferrable = &yes
+	case "initially deferred", "initially_deferred":
+		constraint.InitiallyDeferred = &yes
+	}
+}
+
 // parseTableRef parses a table reference like "schema.table.(column1, column2)" or "schema"."table"."column"
 func (r *Reader) parseTableRef(ref string) (schema, table string, columns []string) {
 	// Extract columns if present in parentheses format