fix: add pull_request trigger to CI workflow so tests gate all PRs

The CI workflow previously only triggered on push to master, meaning PRs
were not tested before merge. Add pull_request trigger for the test job
while restricting the Docker build+push job to push events only.

Closes leeworks-agents/gitea-mobile#204

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

.gitea/workflows/build.yaml

@@ -4,6 +4,9 @@ on:
   push:
     branches:
       - master
+  pull_request:
+    branches:
+      - master
 
 jobs:
   test:
@@ -24,6 +27,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     needs: test
+    if: gitea.event_name == 'push'
     steps:
       - uses: actions/checkout@v4