Init world. Basic registry API server

Work in progress .
This commit adds the core registry service implementation with methods to:
- Retrieve all registry entries
- List entries with cursor-based pagination
- Fetch specific server details by ID

internal/database/database.go

@@ -0,0 +1,48 @@
+package database
+
+import (
+	"context"
+	"errors"
+
+	"github.com/modelcontextprotocol/registry/internal/model"
+)
+
+// Common database errors
+var (
+	ErrNotFound      = errors.New("record not found")
+	ErrAlreadyExists = errors.New("record already exists")
+	ErrInvalidInput  = errors.New("invalid input")
+	ErrDatabase      = errors.New("database error")
+)
+
+// Database defines the interface for database operations on MCPRegistry entries
+type Database interface {
+	// List retrieves all MCPRegistry entries with optional filtering
+	List(ctx context.Context, filter map[string]interface{}, cursor string, limit int) ([]*model.Entry, string, error)
+	// GetByID retrieves a single ServerDetail by it's ID
+	GetByID(ctx context.Context, id string) (*model.ServerDetail, error)
+	// Close closes the database connection
+	Close() error
+}
+
+// ConnectionType represents the type of database connection
+type ConnectionType string
+
+const (
+	// ConnectionTypeMemory represents an in-memory database connection
+	ConnectionTypeMemory ConnectionType = "memory"
+	// ConnectionTypeMongoDB represents a MongoDB database connection
+	ConnectionTypeMongoDB ConnectionType = "mongodb"
+)
+
+// ConnectionInfo provides information about the database connection
+type ConnectionInfo struct {
+	// Type indicates the type of database connection
+	Type ConnectionType
+	// IsConnected indicates whether the database is currently connected
+	IsConnected bool
+	// Raw provides access to the underlying connection object, which will vary by implementation
+	// For MongoDB, this will be *mongo.Client
+	// For MemoryDB, this will be map[string]*model.MCPRegistry
+	Raw any
+}