Merge upstream paullouisageneau/violet (v1.6.1 libjuice, up to 0.5.4)

[PierreFouquet]

Merges the latest from upstream paullouisageneau/violet into our fork.

What this brings in

• libjuice → v1.6.1 (submodule bump eff89f7 → b509b6c), plus several intermediate libjuice updates.
• Upstream CMake fixes (C standard, FindLibJuice module, deprecation warning) and version bumps.
• Misc upstream Docker/Readme updates.

Our version was already at 0.5.4, matching upstream — no version change.

Conflict resolution

The only conflict was Dockerfile. Ours and upstream had made identical changes (uppercase AS, -DCMAKE_BUILD_TYPE=Release, ENTRYPOINT over CMD); the sole real difference was the base image.

• Kept our alpine:3.23 over upstream's alpine:3.20.

Net diff vs current master

 Dockerfile    | 2 +-   (keeps alpine:3.23)
 deps/libjuice | 2 +-   (bumped to upstream v1.6.1)

All fork-specific work (build workflows, Alpine bump) is preserved.

🤖 Generated with Claude Code

[PierreFouquet]

Deployment follow-up: babblevoice/babble-configs#111 bumps the stunserver image to 0.5.4 and fixes the compose command: for the new ENTRYPOINT (drops the leading violet token that would otherwise be passed as a stray non-option arg and crash-loop the container). Publish the tinpotnick/violet:0.5.4 image after this merges, then merge #111.