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/css/views/inlineViewer.css

@@ -212,6 +212,54 @@
   tab-size: 4;
 }
 
+/* Video viewer */
+.inline-viewer-video {
+  max-width: 100%;
+  max-height: 100%;
+  object-fit: contain;
+  border-radius: 4px;
+  background-color: #000;
+}
+
+/* Audio viewer */
+.inline-viewer-audio-wrapper {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 24px;
+  padding: 48px 32px;
+  max-width: 500px;
+  width: 100%;
+}
+
+.inline-viewer-audio-icon {
+  font-size: 80px;
+  color: #94a3b8;
+  animation: audio-pulse 2s ease-in-out infinite;
+}
+
+@keyframes audio-pulse {
+  0%, 100% { opacity: 0.6; transform: scale(1); }
+  50% { opacity: 1; transform: scale(1.05); }
+}
+
+.inline-viewer-audio-name {
+  font-size: 16px;
+  font-weight: 500;
+  color: #475569;
+  text-align: center;
+  word-break: break-word;
+  max-width: 100%;
+}
+
+.inline-viewer-audio {
+  width: 100%;
+  max-width: 460px;
+  border-radius: 8px;
+  outline: none;
+}
+
 /* Responsive adjustments */
 @media (max-width: 768px) {
   .inline-viewer-content {