feat: P1 — audio/video modal player + MIME detection via magic bytes (infer)

Backend:
- Add infer crate for magic-byte MIME detection (<1μs per file)
- New src/common/mime_detect.rs: refine_content_type() with priority
  magic bytes > extension > client Content-Type
- Inject MIME refinement in file upload handler (after spool to temp)
- Inject MIME refinement in chunked upload handler (after assembly)

Frontend:
- Extend isViewableFile() to include audio/* and video/*
- Add createMediaViewer() to InlineViewer with <audio>/<video> controls
- Blob URL pattern for authenticated streaming playback
- Graceful fallback for unsupported codecs (error message + download)
- CSS: video player, audio wrapper with animated icon, responsive

static/js/app/uiFileTypes.js

@@ -8,6 +8,8 @@ const uiFileTypes = {
         if (!file || !file.mime_type) return false;
         if (file.mime_type.startsWith('image/')) return true;
         if (file.mime_type === 'application/pdf') return true;
+        if (file.mime_type.startsWith('audio/')) return true;
+        if (file.mime_type.startsWith('video/')) return true;
         return window.isTextViewable ? window.isTextViewable(file.mime_type) : false;
     },