docker: fail-closed rebuild, ffmpeg checksum, smaller runtime

- Replace the mtime-touch stub-rebuild hack with cargo clean -p (a
  future-dated host file could silently ship the stub binary)
- Optional FFMPEG_SHA256 build arg verified before extraction
- Drop the redundant libssl3/libcrypto copies and the root
  supplementary group; strip the release binary; add a webhook-mode
  healthcheck to the compose example

Cargo.toml

@@ -1,3 +1,7 @@
 [workspace]
 members = ["crates/x-media", "crates/xmedia-bot"]
 resolver = "3"
+
+# Smaller production binary; debug symbols are not shipped anyway.
+[profile.release]
+strip = true