fix(deploy): pin backend Dockerfiles' builder/runtime base to the same Debian release

accounts-service crash-looped in production: GLIBC_2.38 not found. The
builder stage used the floating rust:1-slim tag (now Debian trixie,
glibc 2.38+) while the runtime stage used debian:bookworm-slim (glibc
2.36) — a base mismatch. Pin both stages to trixie so the runtime glibc
is always at least as new as what the binary was linked against.

Claude-Session: https://claude.ai/code/session_01F1M1Jic1wTSn4igUENynmZ

backend/accounts-service/Dockerfile

@@ -1,5 +1,5 @@
 # Build context must be backend/ (the workspace root).
-FROM rust:1-slim AS builder
+FROM rust:1-trixie AS builder
 RUN apt-get update && apt-get install -y --no-install-recommends \
     protobuf-compiler pkg-config libssl-dev ca-certificates \
     && rm -rf /var/lib/apt/lists/*
@@ -11,7 +11,7 @@ COPY gateway ./gateway
 COPY configs-service ./configs-service
 RUN cargo build --release -p accounts-service
 
-FROM debian:bookworm-slim
+FROM debian:trixie-slim
 RUN apt-get update && apt-get install -y --no-install-recommends \
     ca-certificates libssl3 \
     && rm -rf /var/lib/apt/lists/*