feat(migrate): add import CLI command and importer for migration data

Introduce a new `import` subcommand to the server binary that reads
exported JSON files and imports masters, providers, keys, bindings,
and namespaces into the database.

Key features:
- Support for dry-run mode to validate without writing
- Conflict policies: skip existing or overwrite
- Optional binding import via --include-bindings flag
- Auto-generation of master keys with secure hashing
- Namespace auto-creation for referenced namespaces
- Detailed import summary with warnings and created credentials

internal/migrate/schema.go

@@ -0,0 +1,98 @@
+package migrate
+
+import (
+	"encoding/json"
+	"time"
+)
+
+// ExportResult represents the complete export output.
+type ExportResult struct {
+	Version  string   `json:"version"`
+	Source   Source   `json:"source"`
+	Data     Data     `json:"data"`
+	Warnings []string `json:"warnings,omitempty"`
+}
+
+// Source represents the source system information.
+type Source struct {
+	Type       string    `json:"type"`
+	Version    string    `json:"version"`
+	ExportedAt time.Time `json:"exported_at"`
+}
+
+// Data contains all exported entities.
+type Data struct {
+	Providers []Provider `json:"providers,omitempty"`
+	Masters   []Master   `json:"masters,omitempty"`
+	Keys      []Key      `json:"keys,omitempty"`
+	Bindings  []Binding  `json:"bindings,omitempty"`
+}
+
+// Provider represents an EZ-API provider (mapped from New API channel).
+type Provider struct {
+	OriginalID   int      `json:"original_id"`
+	Name         string   `json:"name"`
+	Type         string   `json:"type"`
+	BaseURL      string   `json:"base_url,omitempty"`
+	APIKey       string   `json:"api_key"`
+	Models       []string `json:"models,omitempty"`
+	PrimaryGroup string   `json:"primary_group"`
+	AllGroups    []string `json:"all_groups,omitempty"`
+	Weight       int      `json:"weight"`
+	Priority     int      `json:"priority,omitempty"`
+	Status       string   `json:"status"`
+	AutoBan      bool     `json:"auto_ban"`
+
+	IsMultiKey    bool   `json:"is_multi_key,omitempty"`
+	MultiKeyIndex int    `json:"multi_key_index,omitempty"`
+	OriginalName  string `json:"original_name,omitempty"`
+
+	Original json.RawMessage `json:"_original,omitempty"`
+}
+
+// Master represents an EZ-API master (inferred from New API user).
+type Master struct {
+	Name             string   `json:"name"`
+	Group            string   `json:"group"`
+	Namespaces       []string `json:"namespaces,omitempty"`
+	DefaultNamespace string   `json:"default_namespace,omitempty"`
+	MaxChildKeys     int      `json:"max_child_keys,omitempty"`
+	GlobalQPS        int      `json:"global_qps,omitempty"`
+	Status           string   `json:"status"`
+
+	SourceUserID int    `json:"_source_user_id"`
+	SourceEmail  string `json:"_source_email,omitempty"`
+}
+
+// Key represents an EZ-API key (mapped from New API token).
+type Key struct {
+	MasterRef     string `json:"master_ref"`
+	OriginalToken string `json:"original_token"`
+	Group         string `json:"group,omitempty"`
+	Status        string `json:"status"`
+
+	Scopes     []string `json:"scopes,omitempty"`
+	Namespaces []string `json:"namespaces,omitempty"`
+
+	ModelLimitsEnabled bool     `json:"model_limits_enabled,omitempty"`
+	ModelLimits        []string `json:"model_limits,omitempty"`
+
+	ExpiresAt *time.Time `json:"expires_at,omitempty"`
+
+	AllowIPs []string `json:"allow_ips,omitempty"`
+
+	QuotaLimit     *int64 `json:"quota_limit,omitempty"`
+	QuotaUsed      *int64 `json:"quota_used,omitempty"`
+	UnlimitedQuota bool   `json:"unlimited_quota,omitempty"`
+
+	OriginalID              int  `json:"_original_id"`
+	TokenPlaintextAvailable bool `json:"_token_plaintext_available,omitempty"`
+}
+
+// Binding represents an EZ-API binding (optional, from abilities).
+type Binding struct {
+	Namespace  string `json:"namespace"`
+	RouteGroup string `json:"route_group"`
+	Model      string `json:"model"`
+	Status     string `json:"status"`
+}