fix(docker): unblock image builds on this host

- backend dep-cache stage stubs only main.rs/lib.rs, but Cargo.toml
  declares a second [[bin]] crawler at src/bin/crawler.rs, so
  `cargo build --locked` aborts ("can't find bin crawler"). Stub it too.
- runtime was debian:bookworm-slim (glibc 2.36) while rust:1-slim now
  tracks trixie (glibc 2.41) -> "GLIBC_2.39 not found" at boot. Pin the
  runtime to debian:trixie-slim so it matches the builder's glibc.
- frontend healthcheck probed localhost (-> musl picks IPv6 ::1) but the
  Node server binds IPv4 0.0.0.0 only -> false "unhealthy". Probe 127.0.0.1.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/Dockerfile

@@ -24,10 +24,12 @@ COPY --from=builder --chown=node:node /app/package.json ./
 USER node
 EXPOSE 3000
 
-# Alpine's busybox `wget` is the canonical lightweight HTTP probe.
-# `--spider` doesn't follow redirects; `node build` serves a 200 on
-# `/` for the homepage so this works without a dedicated /health.
+# Alpine's busybox `wget` is the canonical lightweight HTTP probe. Probe
+# 127.0.0.1, not `localhost`: musl resolves `localhost` to IPv6 ::1 first,
+# but the Node server binds IPv4 0.0.0.0 only, so a localhost probe gets
+# "connection refused" and the container is wrongly marked unhealthy. Use a
+# GET (`-O /dev/null`) since `node build` serves 200 on `/`.
 HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
-  CMD wget -q --spider http://localhost:3000/ || exit 1
+  CMD wget -q -O /dev/null http://127.0.0.1:3000/ || exit 1
 
 CMD ["node", "build"]