Add file visibility (PRIVATE/PUBLIC) + public files API

Adds a visibility enum to files with PRIVATE (default) and PUBLIC states.
PUBLIC files are accessible via an unauthenticated /api/files/v1/{fileID}
endpoint that redirects to a presigned S3 URL. Introduces pkg/file service
to manage file operations. Logo uploads (trust centers, organizations,
frameworks, references) are marked PUBLIC; other files are PRIVATE.
Includes database migration and backfill for existing logos.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/probod/probod.go

@@ -57,6 +57,7 @@ import (
 	"go.probo.inc/probo/pkg/mailer"
 	"go.probo.inc/probo/pkg/mailman"
 	"go.probo.inc/probo/pkg/probo"
+	"go.probo.inc/probo/pkg/file"
 	"go.probo.inc/probo/pkg/securecookie"
 	"go.probo.inc/probo/pkg/server"
 	"go.probo.inc/probo/pkg/slack"
@@ -473,11 +474,14 @@ func (impl *Implm) Run(
 		slackService,
 	)
 
+	fileService := file.NewService(pgClient, fileManagerService)
+
 	serverHandler, err := server.NewServer(
 		server.Config{
 			AllowedOrigins:    impl.cfg.Api.Cors.AllowedOrigins,
 			ExtraHeaderFields: impl.cfg.Api.ExtraHeaderFields,
 			Probo:             proboService,
+			File:              fileService,
 			IAM:               iamService,
 			Trust:             trustService,
 			ESign:             esignService,