fix(ci): add workflow_dispatch and concurrency guard to auto-tag

Gitea 1.22 silently drops a push event for a workflow when a run for that
same workflow+branch is already in progress. This caused the PR#9 merge to
master to produce no auto-tag run.

- workflow_dispatch: allows manual triggering via API when an event is dropped
- concurrency group (cancel-in-progress: false): causes Gitea to queue a second
  run rather than discard it when one is already active

.gitea/workflows/auto-tag.yml

@@ -2,11 +2,17 @@ name: Auto Tag
 
 # Runs on every merge to master — reads the latest semver tag, increments
 # the patch version, pushes a new tag, then runs release builds in this workflow.
+# workflow_dispatch allows manual triggering when Gitea drops a push event.
 
 on:
   push:
     branches:
       - master
+  workflow_dispatch:
+
+concurrency:
+  group: auto-tag-master
+  cancel-in-progress: false
 
 jobs:
   autotag: