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

Cargo.lock

@@ -366,6 +366,17 @@ dependencies = [
  "shlex",
 ]
 
+[[package]]
+name = "cfb"
+version = "0.7.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d38f2da7a0a2c4ccf0065be06397cc26a81f4e528be095826eee9d4adbb8c60f"
+dependencies = [
+ "byteorder",
+ "fnv",
+ "uuid",
+]
+
 [[package]]
 name = "cfg-if"
 version = "1.0.4"
@@ -1416,6 +1427,15 @@ dependencies = [
  "hashbrown 0.16.1",
 ]
 
+[[package]]
+name = "infer"
+version = "0.19.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a588916bfdfd92e71cacef98a63d9b1f0d74d6599980d11894290e7ddefffcf7"
+dependencies = [
+ "cfb",
+]
+
 [[package]]
 name = "inout"
 version = "0.1.4"
@@ -1838,6 +1858,7 @@ dependencies = [
  "http-range-header",
  "hyper",
  "image",
+ "infer",
  "jsonwebtoken",
  "lru",
  "md5",