Clarify metadata command supports video/audio, drop misfiring format warning (#33)

The `metadata` command handles more than images: `remove_ai_metadata` strips
C2PA / AIGC provenance from MP4/MOV/M4V/M4A and from WebM/MP3/WAV/FLAC/OGG via
ffmpeg. But the help said "from images" and the shared `_validate_image` call
printed "Warning: .mp4 may not be supported" on exactly those supported
containers. The argument's `exists=True` already enforces the file exists, so
the validation call only added the wrong warning here.

Update the docstring to list the real format coverage and drop the
image-only validation from this command. The image commands keep it.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/remove_ai_watermarks/cli.py

@@ -483,14 +483,20 @@ def cmd_metadata(
     output: Path | None,
     keep_standard: bool,
 ) -> None:
-    """Check or remove AI-generation metadata from images.
+    """Check or remove AI-generation metadata (images, video, and audio).
 
-    Strips EXIF AI tags, PNG text chunks, and C2PA provenance manifests.
+    Strips EXIF AI tags, PNG text chunks, C2PA provenance manifests, and the
+    China TC260 AIGC label. Beyond images (PNG/JPEG/WebP/AVIF/HEIF/JXL) it also
+    strips provenance metadata from MP4/MOV/M4V/M4A containers and, via ffmpeg,
+    from WebM/MP3/WAV/FLAC/OGG. The coded image, audio, and video data are left
+    untouched.
     """
     from remove_ai_watermarks.metadata import get_ai_metadata, has_ai_metadata, remove_ai_metadata
 
+    # No _validate_image() here: unlike the image-only commands, metadata also
+    # accepts video/audio containers, so the image-format warning would misfire.
+    # click's `exists=True` on the argument already enforces the file exists.
     _banner()
-    source = _validate_image(source)
 
     if check or (not remove):
         has_ai = has_ai_metadata(source)