Coordinators now dispatch per-OS builds via API

Previously per-OS build workflows triggered on tag push events, but
Gitea doesn't fire events for tags pushed by other workflows. Now:

- release.yml dispatches build-app-{linux,windows,macos}.yml via
  the Gitea API after creating the tag and release
- sidecar-release.yml dispatches build-sidecar-{linux,windows,macos}.yml

Per-OS workflows changed from push+dispatch triggers to dispatch-only
with tag as a required input. To re-run a failed build for the same
version, just dispatch the specific OS workflow with the same tag --
upload logic replaces existing assets automatically.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-app-linux.yml

@@ -1,14 +1,11 @@
 name: Build App (Linux)
 
 on:
-  push:
-    tags:
-      - 'v*'
   workflow_dispatch:
     inputs:
       tag:
         description: 'Release tag to build (e.g. v1.4.5)'
-        required: false
+        required: true
 
 jobs:
   build-linux:
@@ -20,11 +17,8 @@ jobs:
       - name: Determine tag
         id: tag
         run: |
-          if [ -n "${{ github.event.inputs.tag }}" ]; then
-            TAG="${{ github.event.inputs.tag }}"
-          elif [[ "${{ github.ref }}" == refs/tags/* ]]; then
-            TAG="${{ github.ref_name }}"
-          else
+          TAG="${{ github.event.inputs.tag }}"
+          if [ -z "$TAG" ]; then
             TAG=$(git ls-remote --tags --sort=-v:refname origin 'refs/tags/v*' | head -1 | sed 's|.*refs/tags/||')
           fi
           echo "Building for tag: ${TAG}"