refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/infra/database/plugin.go

@@ -0,0 +1,101 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package database provides the relational database infrastructure plugin for Cordis.
+package database
+
+import (
+	"context"
+
+	"github.com/Rain-kl/Wavelet/backend/core"
+	"github.com/Rain-kl/Wavelet/backend/core/contracts"
+	"gorm.io/gorm"
+)
+
+// Option configures the database plugin.
+type Option func(*Plugin)
+
+// WithDB configures an explicit *gorm.DB instance for the plugin.
+func WithDB(d *gorm.DB) Option {
+	return func(p *Plugin) {
+		p.db = d
+	}
+}
+
+// WithNamedDB registers a named secondary database connection.
+func WithNamedDB(name string, d *gorm.DB) Option {
+	return func(p *Plugin) {
+		if p.namedDBs == nil {
+			p.namedDBs = make(map[string]*gorm.DB)
+		}
+		p.namedDBs[name] = d
+	}
+}
+
+// Plugin implements core.Plugin to provide contracts.DBService into the Cordis micro-kernel.
+type Plugin struct {
+	db       *gorm.DB
+	namedDBs map[string]*gorm.DB
+}
+
+// New creates a new database infrastructure plugin.
+func New(opts ...Option) *Plugin {
+	p := &Plugin{
+		namedDBs: make(map[string]*gorm.DB),
+	}
+	for _, opt := range opts {
+		if opt != nil {
+			opt(p)
+		}
+	}
+	return p
+}
+
+// Name returns the unique identifier of the database plugin.
+func (p *Plugin) Name() string {
+	return "database"
+}
+
+// Apply mounts the database service into the Context.
+func (p *Plugin) Apply(ctx *core.Context) error {
+	targetDB := p.db
+	if targetDB == nil {
+		targetDB = DB(context.Background())
+	}
+
+	svc := &dbServiceImpl{
+		primary:  targetDB,
+		namedDBs: p.namedDBs,
+	}
+
+	core.Provide[contracts.DBService](ctx, svc)
+	return nil
+}
+
+type dbServiceImpl struct {
+	primary  *gorm.DB
+	namedDBs map[string]*gorm.DB
+}
+
+func (s *dbServiceImpl) GORM() *gorm.DB {
+	if s.primary != nil {
+		return s.primary
+	}
+	return DB(context.Background())
+}
+
+func (s *dbServiceImpl) DB(ctx context.Context) *gorm.DB {
+	if s.primary != nil {
+		return s.primary.WithContext(ctx)
+	}
+	return DB(ctx)
+}
+
+func (s *dbServiceImpl) Named(name string) *gorm.DB {
+	if s.namedDBs != nil {
+		if d, ok := s.namedDBs[name]; ok && d != nil {
+			return d
+		}
+	}
+	return s.GORM()
+}