feat(pgsql): ✨ Add templates for constraints and sequences

* Introduce new templates for creating unique, check, and foreign key constraints with existence checks.
* Add templates for setting sequence values and creating sequences.
* Refactor existing SQL generation logic to utilize new templates for better maintainability and readability.
* Ensure identifiers are properly quoted to handle special characters and reserved keywords.

pkg/writers/pgsql/migration_writer_test.go

@@ -215,3 +215,70 @@ func TestTemplateExecutor_AuditFunction(t *testing.T) {
 		t.Error("SQL missing DELETE handling")
 	}
 }
+
+func TestWriteMigration_NumericConstraintNames(t *testing.T) {
+	// Current database (empty)
+	current := models.InitDatabase("testdb")
+	currentSchema := models.InitSchema("entity")
+	current.Schemas = append(current.Schemas, currentSchema)
+
+	// Model database (with constraint starting with number)
+	model := models.InitDatabase("testdb")
+	modelSchema := models.InitSchema("entity")
+
+	// Create individual_actor_relationship table
+	table := models.InitTable("individual_actor_relationship", "entity")
+	idCol := models.InitColumn("id", "individual_actor_relationship", "entity")
+	idCol.Type = "integer"
+	idCol.IsPrimaryKey = true
+	table.Columns["id"] = idCol
+
+	actorIDCol := models.InitColumn("actor_id", "individual_actor_relationship", "entity")
+	actorIDCol.Type = "integer"
+	table.Columns["actor_id"] = actorIDCol
+
+	// Add constraint with name starting with number
+	constraint := &models.Constraint{
+		Name:              "215162_fk_actor",
+		Type:              models.ForeignKeyConstraint,
+		Columns:           []string{"actor_id"},
+		ReferencedSchema:  "entity",
+		ReferencedTable:   "actor",
+		ReferencedColumns: []string{"id"},
+		OnDelete:          "CASCADE",
+		OnUpdate:          "NO ACTION",
+	}
+	table.Constraints["215162_fk_actor"] = constraint
+
+	modelSchema.Tables = append(modelSchema.Tables, table)
+	model.Schemas = append(model.Schemas, modelSchema)
+
+	// Generate migration
+	var buf bytes.Buffer
+	writer, err := NewMigrationWriter(&writers.WriterOptions{})
+	if err != nil {
+		t.Fatalf("Failed to create writer: %v", err)
+	}
+	writer.writer = &buf
+
+	err = writer.WriteMigration(model, current)
+	if err != nil {
+		t.Fatalf("WriteMigration failed: %v", err)
+	}
+
+	output := buf.String()
+	t.Logf("Generated migration:\n%s", output)
+
+	// Verify constraint name is properly quoted
+	if !strings.Contains(output, `"215162_fk_actor"`) {
+		t.Error("Constraint name starting with number should be quoted")
+	}
+
+	// Verify the SQL is syntactically correct (contains required keywords)
+	if !strings.Contains(output, "ADD CONSTRAINT") {
+		t.Error("Migration missing ADD CONSTRAINT")
+	}
+	if !strings.Contains(output, "FOREIGN KEY") {
+		t.Error("Migration missing FOREIGN KEY")
+	}
+}