feat(release): bundle ffmpeg + ffprobe in tarballs and Docker image

Operators no longer have to install ffmpeg manually. Both the release
tarballs (5 platforms × 2 binaries) and the Docker image now ship a
working ffmpeg + ffprobe pair adjacent to the unarr binary;
ResolveFFmpeg / ResolveFFprobe pick them up via the "adjacent to
executable" branch with zero configuration.

Tarball bundle (scripts/download-ffmpeg-static.sh + .goreleaser.yml):
- ffbinaries.com (johnvansickle / Zeranoe-style static GPL builds) for
  linux-amd64, linux-arm64, darwin-amd64, windows-amd64
- evermeet.cx universal Mach-O for darwin-arm64 (ffbinaries lacks it)
- BtbN/FFmpeg-Builds for windows-arm64 (ffbinaries lacks it)
- Idempotent fetch with curl --retry 5 so transient github.com SSL
  errors don't fail the goreleaser before-hook
- New `before.hooks` runs the script automatically per release; archive
  files glob `dist-ffbinaries/{{ .Os }}-{{ .Arch }}/*` + strip_parent
- Migrated to non-deprecated `formats: [tar.gz]` / `formats: [zip]`
- Verified via `goreleaser release --snapshot --clean --skip=publish` —
  6 archives all carry ffmpeg + ffprobe (~60-130MB each)

Docker image (Dockerfile):
- Replaced the failing BtbN static glibc binaries with Alpine's native
  musl `apk add ffmpeg`. The static GPL builds need glibc + libmvec /
  libgcc_s; gcompat alone is not enough (vector-math symbols unresolved).
  Alpine ships ffmpeg 6.1.2 which is fine for the WebRTC transcoder.
- Image size 174MB, built + ffmpeg/ffprobe/unarr smoke OK.

Targets the v0.8 unarr release (per user direction — new feature, not
a patch). dist-ffbinaries/ added to .gitignore.

scripts/download-ffmpeg-static.sh

@@ -0,0 +1,117 @@
+#!/usr/bin/env bash
+# scripts/download-ffmpeg-static.sh — fetch static ffmpeg + ffprobe binaries
+# for every platform we ship. Run by goreleaser's `before.hooks` so each
+# tarball can bundle the binaries adjacent to `unarr`.
+#
+# Source: https://ffbinaries.com (same index the runtime fallback uses).
+# Output:
+#   dist-ffbinaries/<goos>-<goarch>/{ffmpeg, ffprobe}[.exe]
+# Idempotent: skips downloads when the target file already exists.
+
+set -euo pipefail
+
+# Map ffbinaries platform key → goreleaser {Os}-{Arch}. ffbinaries.com only
+# ships an x86_64 macOS build; for darwin-arm64 we fall back to evermeet.cx
+# universal binaries (handled separately below).
+PLATFORMS=(
+  "linux-64:linux-amd64"
+  "linux-arm64:linux-arm64"
+  "osx-64:darwin-amd64"
+  "windows-64:windows-amd64"
+)
+DEST_ROOT="${FFBINARIES_DEST:-dist-ffbinaries}"
+INDEX_URL="https://ffbinaries.com/api/v1/version/latest"
+
+for cmd in curl jq unzip; do
+  command -v "$cmd" >/dev/null 2>&1 || {
+    echo "[ffbin] missing required tool: $cmd" >&2
+    exit 2
+  }
+done
+
+mkdir -p "$DEST_ROOT"
+
+echo "[ffbin] fetching index from $INDEX_URL"
+INDEX_JSON="$(curl -fsSL "$INDEX_URL")"
+VERSION="$(echo "$INDEX_JSON" | jq -r .version)"
+echo "[ffbin] ffbinaries version: $VERSION"
+
+for entry in "${PLATFORMS[@]}"; do
+  ffbkey="${entry%%:*}"
+  goplat="${entry##*:}"
+  outdir="$DEST_ROOT/$goplat"
+  mkdir -p "$outdir"
+
+  for tool in ffmpeg ffprobe; do
+    binname="$tool"
+    [[ "$goplat" == windows-* ]] && binname="${tool}.exe"
+
+    if [ -f "$outdir/$binname" ]; then
+      echo "[ffbin] skip $goplat/$binname (already present)"
+      continue
+    fi
+
+    url="$(echo "$INDEX_JSON" | jq -r ".bin[\"$ffbkey\"][\"$tool\"] // empty")"
+    if [ -z "$url" ]; then
+      echo "[ffbin] WARN $goplat/$tool: no download URL in index" >&2
+      continue
+    fi
+
+    tmpzip="$(mktemp --suffix=.zip)"
+    echo "[ffbin] fetch $goplat/$tool from $url"
+    curl -fsSL --retry 5 --retry-delay 3 --retry-all-errors "$url" -o "$tmpzip"
+    unzip -p "$tmpzip" "$binname" > "$outdir/$binname"
+    chmod +x "$outdir/$binname"
+    rm -f "$tmpzip"
+  done
+done
+
+# --- darwin-arm64 via evermeet.cx (universal binary; ffbinaries lacks it) ---
+darwin_arm_dir="$DEST_ROOT/darwin-arm64"
+mkdir -p "$darwin_arm_dir"
+for tool in ffmpeg ffprobe; do
+  out="$darwin_arm_dir/$tool"
+  if [ -f "$out" ]; then
+    echo "[ffbin] skip darwin-arm64/$tool (already present)"
+    continue
+  fi
+  url="https://evermeet.cx/ffmpeg/getrelease/$tool/zip"
+  tmpzip="$(mktemp --suffix=.zip)"
+  echo "[ffbin] fetch darwin-arm64/$tool from $url"
+  curl -fsSL --retry 5 --retry-delay 3 --retry-all-errors "$url" -o "$tmpzip"
+  unzip -p "$tmpzip" "$tool" > "$out"
+  chmod +x "$out"
+  rm -f "$tmpzip"
+done
+
+# --- windows-arm64 via BtbN/FFmpeg-Builds (ffbinaries lacks it) ---
+# BtbN ships a single zip per platform with ffmpeg.exe + ffprobe.exe under
+# ffmpeg-master-latest-winarm64-gpl/bin/. Extract both in one fetch.
+win_arm_dir="$DEST_ROOT/windows-arm64"
+mkdir -p "$win_arm_dir"
+needs_win_arm=0
+for tool in ffmpeg.exe ffprobe.exe; do
+  [ -f "$win_arm_dir/$tool" ] || needs_win_arm=1
+done
+if [ "$needs_win_arm" = "1" ]; then
+  url="https://github.com/BtbN/FFmpeg-Builds/releases/download/latest/ffmpeg-master-latest-winarm64-gpl.zip"
+  tmpzip="$(mktemp --suffix=.zip)"
+  echo "[ffbin] fetch windows-arm64/{ffmpeg,ffprobe}.exe from $url"
+  curl -fsSL --retry 5 --retry-delay 3 --retry-all-errors "$url" -o "$tmpzip"
+  for tool in ffmpeg.exe ffprobe.exe; do
+    out="$win_arm_dir/$tool"
+    member="$(unzip -Z1 "$tmpzip" "*/bin/$tool" 2>/dev/null | head -1)"
+    if [ -z "$member" ]; then
+      echo "[ffbin] WARN windows-arm64/$tool: not found in BtbN zip" >&2
+      continue
+    fi
+    unzip -p "$tmpzip" "$member" > "$out"
+    chmod +x "$out"
+  done
+  rm -f "$tmpzip"
+else
+  echo "[ffbin] skip windows-arm64 (already present)"
+fi
+
+echo "[ffbin] done. layout:"
+find "$DEST_ROOT" -type f -printf "  %p (%s bytes)\n"