Optimize build: Kaniko cache flags, split Dockerfile stages, install-tools script

- Kaniko: --cache-ttl=168h, --snapshot-mode=redo, --compressed-caching=false
- Dockerfile: Split server-build into server-deps + server-build for better layer caching
- Replace rm+reinstall node_modules with npm prune --omit=dev
- Move ffmpeg/yt-dlp install to tools/install-tools.sh (single RUN layer)
- Remove separate ffmpeg-fetch stage and inline curl/yt-dlp install
- Remove $CI_COMMIT_SHA tag destination (unused, saves push time)
- bump-version: alpine/git image instead of alpine + apk add git

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tools/install-tools.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+set -e
+
+echo "[install-tools] Installing ffmpeg + yt-dlp..."
+
+apt-get update && apt-get install -y --no-install-recommends curl ca-certificates xz-utils \
+  && rm -rf /var/lib/apt/lists/*
+
+# FFmpeg (static build from yt-dlp project)
+curl -L https://github.com/yt-dlp/FFmpeg-Builds/releases/download/latest/ffmpeg-master-latest-linux64-gpl.tar.xz \
+  -o /tmp/ffmpeg.tar.xz
+mkdir -p /tmp/ffmpeg
+tar -xJf /tmp/ffmpeg.tar.xz -C /tmp/ffmpeg --strip-components=2 --wildcards "*/bin/ffmpeg"
+mv /tmp/ffmpeg/ffmpeg /usr/local/bin/ffmpeg
+chmod +x /usr/local/bin/ffmpeg
+rm -rf /tmp/ffmpeg /tmp/ffmpeg.tar.xz
+
+# yt-dlp (standalone linux binary, no Python needed)
+curl -L https://github.com/yt-dlp/yt-dlp/releases/latest/download/yt-dlp_linux \
+  -o /usr/local/bin/yt-dlp
+chmod +x /usr/local/bin/yt-dlp
+
+# Cleanup curl/xz (keep ca-certificates for HTTPS)
+apt-get purge -y curl xz-utils && apt-get autoremove -y && rm -rf /var/lib/apt/lists/*
+
+echo "[install-tools] Done. ffmpeg=$(ffmpeg -version 2>&1 | head -1), yt-dlp=$(yt-dlp --version 2>/dev/null || echo 'ok')"