fix(installer): add src/ deploy + drop MakeDirectory=yes; bump 1.3.1

- install.sh now deploys the src/ directory alongside routes/.
  Without this, fresh installs of v1.3.0+ produce containers whose
  Dockerfile references src/ but the directory is missing on the host
  filesystem, so docker build fails with "/src: not found".

- The fallback heredoc that writes /etc/systemd/system/dashcaddy-
  updater.path drops MakeDirectory=yes for the same reason it was
  removed from the on-disk unit (e994ad1): systemd creates the watched
  trigger.json path as an empty directory on unit start, blocking
  every subsequent update with EISDIR.

Bumped to 1.3.1 so the existing v1.3.0 instance auto-updates and
picks up these and the host-script fix from 0cf6323.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dashcaddy-installer/install.sh

@@ -435,6 +435,7 @@ fetch_source() {
   cp -f "${api_src}/Dockerfile" "$API_DIR/"
   cp -f "${api_src}/openapi.yaml" "$API_DIR/" 2>/dev/null || true
   [[ -d "${api_src}/routes" ]] && cp -rf "${api_src}/routes" "$API_DIR/"
+  [[ -d "${api_src}/src" ]] && cp -rf "${api_src}/src" "$API_DIR/"
   ok "API files deployed"
 
   # Deploy dashboard files
@@ -698,7 +699,6 @@ install_updater_service() {
 Description=Watch for DashCaddy update trigger
 [Path]
 PathChanged=/opt/dashcaddy/updates/trigger.json
-MakeDirectory=yes
 [Install]
 WantedBy=multi-user.target
 PATHEOF