feat: add FRANCIS_HOST to connect to a standalone Francis runtime

FRANCIS_HOST decides where the Francis actor runtime lives. When it is
empty or set to "embedded" (the default), Pocket ID starts the runtime inside its own process, backed by its own database.

Any other value is the address, or a comma-separated list of addresses, of a standalone Francis runtime. Pocket ID then connects to it as a remote actor host and starts no embedded runtime.

Note: connecting to a standalone runtime also needs FRANCIS_HOST_PSK or FRANCIS_HOST_JWT, and optionally (but recommended) FRANCIS_CA.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DMgoTZtznSjP4SHTaHbRen

backend/internal/ldapsync/module.go

@@ -7,7 +7,7 @@ import (
 	"net/http"
 
 	"github.com/gin-gonic/gin"
-	"github.com/italypaleale/francis/host/local"
+	francishost "github.com/italypaleale/francis/host"
 	"gorm.io/gorm"
 
 	"github.com/pocket-id/pocket-id/backend/internal/appconfig"
@@ -43,7 +43,7 @@ type ScimSyncScheduler interface {
 
 type Dependencies struct {
 	DB          *gorm.DB
-	Actors      *local.Host
+	Actors      francishost.Host
 	HTTPClient  *http.Client
 	FileStorage storage.FileStorage