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>

backend/Dockerfile

@@ -10,7 +10,8 @@ RUN apt-get update \
 # exact crate versions CI tested. Without Cargo.lock + the flag, cargo
 # would silently resolve fresh on every image build.
 COPY Cargo.toml Cargo.lock ./
-RUN mkdir src && echo "fn main() {}" > src/main.rs && echo "" > src/lib.rs \
+RUN mkdir -p src/bin && echo "fn main() {}" > src/main.rs && echo "" > src/lib.rs \
+ && echo "fn main() {}" > src/bin/crawler.rs \
  && cargo build --locked --release \
  && rm -rf src
 
@@ -18,7 +19,10 @@ COPY src ./src
 COPY migrations ./migrations
 RUN touch src/main.rs src/lib.rs && cargo build --locked --release
 
-FROM debian:bookworm-slim
+FROM debian:trixie-slim
+# Runtime base must match the builder's Debian release: `rust:1-slim` tracks
+# trixie (glibc 2.41), so a bookworm runtime (glibc 2.36) can't run the
+# binary ("GLIBC_2.39 not found"). Keep these two in lockstep on bumps.
 # `curl` is for the container HEALTHCHECK; `ca-certificates` is for
 # outbound HTTPS (crawler covers/pages).
 RUN apt-get update \