add file storage: metadata schema, minio upload/download, content-type extension mapping

- add files_metadata and files_metadata_connections tables with CRUD helpers
- add FileMetadata type and Sha256Hash typedef; replace Media struct
- add minio upload, presigned download URL, and key generation
- fix bucket existence check to use FileStorageBucketName instead of hardcoded "main"
- fix files_metadata_connections table name and trailing comma in DDL
- fix column name original -> name in files_metadata schema
- add canonical MIME-to-extension map with .unk fallback
- add FileDownloadLinkTtl constant (24h)

packages/globals/globals.go

@@ -1,6 +1,9 @@
 package globals
 
+import "time"
+
 const (
-	MaxDirectMsgCache     uint32 = 12
-	FileStorageBucketName string = "communicator"
+	MaxDirectMsgCache     uint32        = 12
+	FileStorageBucketName string        = "communicator"
+	FileDownloadLinkTtl   time.Duration = 24 * time.Hour
 )