feat(backend): implement database schema, models, and import workflow

- Add ProxyHost, CaddyConfig, RemoteServer, SSL, AccessList, User, Setting, ImportSession models
- Implement ProxyHostService and RemoteServerService with domain/name uniqueness validation
- Add Caddyfile import handler with conflict detection and user review workflow
- Create ProxyHostHandler and RemoteServerHandler with full CRUD operations
- Wire up Gin/GORM/SQLite dependencies and create missing internal packages
- Add database connection layer, server routing, and version info
- Update routes to register all new handlers and auto-migrate models
- Configure import environment variables and mount points

Addresses Issue #5 (data persistence) and Issue #43 (remote servers)
Backend now compiles cleanly with go build

backend/internal/database/database.go

@@ -0,0 +1,18 @@
+package database
+
+import (
+	"fmt"
+
+	"gorm.io/driver/sqlite"
+	"gorm.io/gorm"
+)
+
+// Connect opens a SQLite database connection.
+func Connect(dbPath string) (*gorm.DB, error) {
+	db, err := gorm.Open(sqlite.Open(dbPath), &gorm.Config{})
+	if err != nil {
+		return nil, fmt.Errorf("open database: %w", err)
+	}
+
+	return db, nil
+}