- 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)
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