refactor: extract ProxyProvider interface with None and NPM implementations

Replace direct npm.Client usage throughout the codebase with the
proxy.Provider interface, enabling pluggable proxy backends. The
deployer, API layer, and proxy manager now use provider-agnostic
route management (ConfigureRoute/DeleteRoute) instead of NPM-specific
API calls. Adds ProxyRouteID (string) to Instance model and
ProxyProvider setting to Settings, with SQLite migrations for
backward compatibility.

internal/store/models.go

@@ -63,6 +63,7 @@ type Settings struct {
 	DNSProvider          string `json:"dns_provider"`
 	CloudflareAPIToken   string `json:"cloudflare_api_token"`
 	CloudflareZoneID     string `json:"cloudflare_zone_id"`
+	ProxyProvider        string `json:"proxy_provider"`
 	BackupEnabled        bool   `json:"backup_enabled"`
 	BackupIntervalHours  int    `json:"backup_interval_hours"`
 	BackupRetentionCount int    `json:"backup_retention_count"`
@@ -97,9 +98,10 @@ type Instance struct {
 	ContainerID string `json:"container_id"`
 	ImageTag    string `json:"image_tag"`
 	Subdomain   string `json:"subdomain"`
-	NpmProxyID  int    `json:"npm_proxy_id"`
-	Status      string `json:"status"` // running, stopped, failed, removing
-	Port        int    `json:"port"`
+	NpmProxyID   int    `json:"npm_proxy_id"`
+	ProxyRouteID string `json:"proxy_route_id"`
+	Status       string `json:"status"` // running, stopped, failed, removing
+	Port         int    `json:"port"`
 	LastAliveAt string `json:"last_alive_at"`
 	CreatedAt   string `json:"created_at"`
 	UpdatedAt   string `json:"updated_at"`