ci: align pipeline on web/mydl pattern

Refactor the 4 trigger workflows to call a shared _ci-common.yaml
(hadolint + build-test), like web/mydl:

- pr.yaml       -> PR checks via _ci-common (concurrency group, dispatch)
- main.yaml     -> CI + cached load-build + git semver tag (PAT)
- tag.yaml      -> CI + build/push :vX.Y.Z and :vX.Y.Z-latest
- cron.yaml     -> nightly rebuild of :vX.Y.Z-latest
- _ci-common.yaml -> hadolint (step-level continue-on-error) + build-test
                     with GHA cache, load:true and tests/test.sh actually run
                     (real HTTP smoke tests - they existed but were never
                     invoked by any workflow)

Fixes: main.yaml tag job had a corrupted remote URL line; tests never ran
on any trigger path.

.gitea/workflows/pr.yaml

@@ -1,24 +1,12 @@
 name: PR Checks
 on:
+  workflow_dispatch:
   pull_request:
     branches: [main]
+concurrency:
+  group: pr-${{ github.ref }}
+  cancel-in-progress: true
 jobs:
-  hadolint:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
-      - uses: hadolint/hadolint-action@06be81baf89a55ffd0e24b8f04a4185738dd3387 # v3.5.0
-        continue-on-error: true
-        with:
-          dockerfile: Dockerfile
-
-  build-test:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
-      - uses: docker/setup-buildx-action@f87e5991a6d7451dcb8d9637bfbc97413f497069 # v4
-      - uses: docker/login-action@dbcb813823bdd20940b903addbd779551569679f # v4
-        with:
-          username: ${{ secrets.DOCKERHUB_USERNAME }}
-          password: ${{ secrets.DOCKERHUB_TOKEN }}
-      - run: docker build -t ci-image:${{ github.sha }} .
+  ci:
+    uses: ./.gitea/workflows/_ci-common.yaml
+    secrets: inherit