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.

.goreleaser.yml

@@ -2,6 +2,14 @@ version: 2
 
 project_name: unarr
 
+# Pre-build hook: fetch static ffmpeg + ffprobe per platform so each
+# release tarball ships them adjacent to the unarr binary. ResolveFFmpeg /
+# ResolveFFprobe pick them up via the "adjacent to executable" branch — no
+# system install or runtime download needed.
+before:
+  hooks:
+    - bash scripts/download-ffmpeg-static.sh
+
 builds:
   - main: ./cmd/unarr/
     binary: unarr
@@ -20,11 +28,21 @@ builds:
       - -X github.com/torrentclaw/unarr/internal/sentry.dsn={{ .Env.SENTRY_DSN }}
 
 archives:
-  - format: tar.gz
+  - formats: [tar.gz]
     name_template: "{{ .ProjectName }}_{{ .Version }}_{{ .Os }}_{{ .Arch }}"
     format_overrides:
       - goos: windows
-        format: zip
+        formats: [zip]
+    files:
+      - LICENSE*
+      - README*
+      # Bundle the matching ffmpeg + ffprobe (filename includes .exe on Windows
+      # because download-ffmpeg-static.sh writes ffmpeg.exe / ffprobe.exe there).
+      - src: "dist-ffbinaries/{{ .Os }}-{{ .Arch }}/*"
+        dst: .
+        strip_parent: true
+        info:
+          mode: 0o755
 
 checksum:
   name_template: "checksums.txt"