fix: add zstd to Docker image and validate required binaries at startup

[alecthomas]

Add zstd to the Alpine package list in the Dockerfile so snapshot operations work out of the box.

Validate that required binaries (git, tar, zstd) are present at startup with clear error messages, rather than failing at runtime.

✓ Built cachew:local
→ Starting cachew at http://localhost:8080 (log-level=info)
INF Constructing disk cache limit-mb=250000 evict-interval=1m0s root=./state/cache max-ttl=8h0m0s
INF Constructing tiered cache tiers=1
DBG Cache backend cache=disk:/app/state/cache
DBG Registered strategy handler strategy=apiv1 pattern="GET /api/v1/object/{key}"
DBG Registered strategy handler strategy=apiv1 pattern="HEAD /api/v1/object/{key}"
DBG Registered strategy handler strategy=apiv1 pattern="POST /api/v1/object/{key}"
DBG Registered strategy handler strategy=apiv1 pattern="DELETE /api/v1/object/{key}"
DBG Registered strategy handler strategy=apiv1 pattern="GET /api/v1/stats"
cachewd: error: load config: /app/cachew.hcl:24:1: zstd is required for snapshots (snapshot-interval > 0) but not found in PATH
error: Recipe `run` failed on line 47 with exit code 1