build(backend): one shared Dockerfile with cargo cache mounts, sequential compose build, stripped release binaries

All four services now share one builder stage (dependencies compile once per
deploy, not once per service). BuildKit cache mounts keep the cargo registry
and target/ between deploys: a one-line change rebuilds in ~16 s instead of
recompiling the whole dependency tree.

backend/Cargo.toml

@@ -11,3 +11,8 @@ members = [
 # Planned members, added when their own implementation plan starts
 # (see backend/STRUCTURE.md and TODO.md Фаза 10 / Подсистемы 1-3):
 #   "addons-registry"   — addon marketplace backend (Подсистема 3)
+
+# Smaller binaries and images; no effect on build time or behaviour.
+[profile.release]
+strip = true
+debug = false