Adds a shared/modules symlink mechanism for optional plugin modules

ModuleOrchestrator already discovers SlpModularCms.Modules.*.dll from disk at
startup via reflection, so this needed no code change — only a persistent
location (mirroring shared/wwwroot-web) that survives release swaps, and a
deploy-scp.yaml step to symlink its contents into each new release before
restart.

.gitea/workflows/deploy-scp.yaml

@@ -132,6 +132,34 @@ jobs:
              rm -rf $RELEASE_DIR/wwwroot/web && \
              ln -s ../../../shared/wwwroot-web $RELEASE_DIR/wwwroot/web"
 
+      # Optional plugin modules (future modular support): a compiled SlpModularCms.Modules.*.dll
+      # dropped into shared/modules survives every release, the same persistence pattern as
+      # shared/wwwroot-web above — except there's no cross-account permission dance here, since
+      # shared/modules lives entirely under gitea-workflow's own tree (unlike wwwroot-web, which
+      # reaches into webadmin's). ModuleOrchestrator.DiscoverModules() already scans its own base
+      # directory on disk for matching assemblies at startup (SlpModularCms.Core/Hosting/
+      # ModuleOrchestrator.cs) and loads whatever it finds via reflection — no code change needed,
+      # only somewhere for the DLL to still be after the next deploy replaces releases/{timestamp}.
+      # `ls ... 2>/dev/null` piped to a `while read` (rather than a bare glob loop) keeps this
+      # POSIX-sh safe and correct when shared/modules is empty, which is the common case today.
+      - name: Link persistent modules
+        env:
+          SSH_USER: ${{ secrets.PI_MAIN_USERNAME }}
+          SSH_PASS: ${{ secrets.PI_MAIN_PASSWORD }}
+          SSH_PORT: ${{ secrets.PI_MAIN_PORT }}
+          SSH_HOST: ${{ secrets.PI_MAIN_ADDRESS }}
+        run: |
+          RELEASE_DIR="${{ inputs.deploy_path }}/releases/${{ steps.release.outputs.timestamp }}"
+          sshpass -p "$SSH_PASS" ssh \
+            -p "$SSH_PORT" \
+            -o StrictHostKeyChecking=no \
+            "$SSH_USER@$SSH_HOST" \
+            "mkdir -p ${{ inputs.deploy_path }}/shared/modules && \
+             ls ${{ inputs.deploy_path }}/shared/modules/*.dll 2>/dev/null | while read -r f; do \
+               name=\$(basename \"\$f\"); \
+               ln -sf \"../../shared/modules/\$name\" \"$RELEASE_DIR/\$name\"; \
+             done"
+
       # Atomic release switch (FR-06, D-27): `ln -sfn` replaces the `current` symlink target in a
       # single filesystem operation, so there is no moment where `current` points at a half-written
       # directory. The process is then restarted so it picks up the new assemblies — a running .NET