feat(deploy): sync a scenario's backend/ into the live workspace root

deploy-scenario.sh now single-sources the application backend: it copies the git-tracked files under [scenario].backend into the workspace root (where .pi/ sits beside them), preserving modes and skipping caches/venvs. It overwrites but never prunes, and never restarts -- it prints a restart reminder when backend files change. README's Application code section updated to match.

scripts/deploy-scenario.sh

@@ -108,6 +108,44 @@ if [ "$APPLY" -eq 1 ] && [ -d "$DIR/workspace/bin" ]; then
   done < <(cd "$DIR/workspace" && find bin -type f -printf '%P\n' 2>/dev/null | sed 's|^|bin/|')
 fi
 
+# ---------------------------------------------------------------------------
+# 1c. Application backend
+#
+# A scenario that owns a service keeps its source under backend/ (set by
+# [scenario].backend). It installs into the workspace root, where .pi/ sits
+# beside it, so the repository is the single source of truth. Only git-tracked
+# files are copied -- build caches and virtualenvs never leak -- and file modes
+# (notably the execute bit) are preserved. This overwrites but never prunes:
+# removing a file the repo no longer tracks from a live tree is too blunt to do
+# unattended. Like every other step it never restarts the service; it prints a
+# reminder, because restarting decides when to interrupt a live conversation.
+# ---------------------------------------------------------------------------
+BACKEND="$(toml_get "$PROFILE" scenario backend)"
+if [ -n "$BACKEND" ]; then
+  case "$BACKEND" in /*) ;; *) BACKEND="$REPO_ROOT/$BACKEND" ;; esac
+  [ -d "$BACKEND" ] || die "profile sets [scenario].backend to '$BACKEND', which does not exist"
+  BACKEND_CHANGES=0
+  while IFS= read -r rel; do
+    src="$BACKEND/$rel"
+    dst="$WORKSPACE/$rel"
+    if [ -f "$dst" ] && cmp -s "$src" "$dst"; then
+      continue
+    fi
+    BACKEND_CHANGES=$((BACKEND_CHANGES + 1))
+    if [ -f "$dst" ]; then info "  update  $rel"; else info "  create  $rel"; fi
+    if [ "$APPLY" -eq 1 ]; then
+      install -d -m 755 "$(dirname "$dst")"
+      install -m "$(stat -c '%a' "$src")" "$src" "$dst"
+    fi
+  done < <(git -C "$BACKEND" ls-files)
+  CHANGES=$((CHANGES + BACKEND_CHANGES))
+  if [ "$BACKEND_CHANGES" -gt 0 ]; then
+    warn "backend: $BACKEND_CHANGES file(s) changed; restart $(toml_get "$PROFILE" scenario service) to apply"
+  else
+    ok "backend already matches"
+  fi
+fi
+
 [ "$CHANGES" -eq 0 ] && ok "workspace already matches"
 
 # ---------------------------------------------------------------------------