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

src/interfaces/api/handlers/chunked_upload_handler.rs

@@ -299,6 +299,14 @@ impl ChunkedUploadHandler {
                 }
             };
 
+        // ── MIME detection (magic bytes + extension fallback) ─────
+        let content_type = crate::common::mime_detect::refine_content_type_from_file(
+            &assembled_path,
+            &filename,
+            &content_type,
+        )
+        .await;
+
         // Upload from assembled file on disk — zero extra RAM copies, hash pre-computed
         match upload_service
             .upload_file_from_path(