task 9: Centralize file extension and media type mappings in internal/mediatype

Create a new internal/mediatype package with a single source of truth for:
- Extension-to-media-type mappings (TypeForExt)
- Extension-to-MIME mappings (MIMETypeForExt)
- Supported extension checks (IsSupportedExt, IsImageExt, IsCoverImageExt)

Replace duplicated logic across:
- internal/service/service.go (supportedExtensions, guessMediaType)
- internal/scanner/scanner.go (mediaExtensions, imageExtensions, mediaTypeFromExt)
- internal/api/handlers.go (probe.MimeTypeForFilename)
- internal/probe/probe.go (imageExtensions, isImagePath)
- internal/probe/remux.go (MimeTypeForFilename)

Divergent defaults unified: both scanner and service now default unknown
extensions to video (model.MediaTypeVideo) via mediatype.TypeForExt.

Update tests to use mediatype package and remove obsolete scanner tests.
Update AGENTS.md to reflect the new package.

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