fix(docker): switch ffprobe download from johnvansickle.com to BtbN/FFmpeg-Builds

johnvansickle.com was unreachable from GitHub Actions runners (2 failed releases),
switching to BtbN static builds on GitHub CDN which are more reliable.

Dockerfile

@@ -1,21 +1,23 @@
 # ---- ffprobe static binary stage ----
-# Download a static ffprobe-only build (~30MB) to avoid the full ffmpeg package (~1GB).
-# johnvansickle.com provides reliable static builds for amd64/arm64.
+# Download a static ffprobe build from BtbN/FFmpeg-Builds (GitHub CDN, reliable).
 FROM alpine:3.22 AS ffprobe-dl
 
 RUN apk add --no-cache curl xz
 
 RUN ARCH=$(uname -m) && \
     case "$ARCH" in \
-      x86_64)  SLUG="amd64" ;; \
-      aarch64) SLUG="arm64" ;; \
+      x86_64)  SLUG="linux64" ;; \
+      aarch64) SLUG="linuxarm64" ;; \
       *) echo "Unsupported arch: $ARCH" && exit 1 ;; \
     esac && \
-    curl -fsSL "https://johnvansickle.com/ffmpeg/releases/ffmpeg-release-${SLUG}-static.tar.xz" -o /tmp/ff.tar.xz && \
-    tar xJ -f /tmp/ff.tar.xz --strip-components=1 -C /tmp/ && \
-    mv /tmp/ffprobe /usr/local/bin/ffprobe && \
+    curl -fsSL --retry 3 --retry-delay 5 \
+      "https://github.com/BtbN/FFmpeg-Builds/releases/download/latest/ffmpeg-master-latest-${SLUG}-gpl.tar.xz" \
+      -o /tmp/ff.tar.xz && \
+    mkdir /tmp/ffbuild && \
+    tar xJ -f /tmp/ff.tar.xz --strip-components=1 -C /tmp/ffbuild/ && \
+    mv /tmp/ffbuild/bin/ffprobe /usr/local/bin/ffprobe && \
     chmod +x /usr/local/bin/ffprobe && \
-    rm -rf /tmp/ff.tar.xz /tmp/ffmpeg /tmp/ffmpeg-* && \
+    rm -rf /tmp/ff.tar.xz /tmp/ffbuild && \
     ffprobe -version | head -1
 
 # ---- Build stage ----