ci: delete overlay deploy branches after merge

Set delete_branch_after_merge:true on the auto-merge POST in both
deploy-dev and deploy-uat so the per-deploy branches in
cartsnitch/infra (ci/deploy-{dev,uat}-${GITHUB_SHA}) are removed
once their overlay image-tag bump lands on main. Without this flag
every successful deploy would leave a branch behind, accumulating
in cartsnitch/infra and making future re-runs of the same SHA
un-actionable from the existing branch name.

Refs CAR-1195 (CTO fix #2).

.gitea/workflows/ci.yml

@@ -570,7 +570,7 @@ jobs:
           MERGE_RESP=$(curl -sS -X POST \
             -H "Authorization: token ${CI_GITEA_TOKEN}" \
             -H "Content-Type: application/json" \
-            -d '{"Do":"merge"}' \
+            -d '{"Do":"merge","delete_branch_after_merge":true}' \
             "https://git.farh.net/api/v1/repos/cartsnitch/infra/pulls/${PR_NUM}/merge")
           MERGED=$(echo "$MERGE_RESP" | jq -r '.merged // false')
           if [ "$MERGED" != "true" ]; then
@@ -696,7 +696,7 @@ jobs:
           MERGE_RESP=$(curl -sS -X POST \
             -H "Authorization: token ${CI_GITEA_TOKEN}" \
             -H "Content-Type: application/json" \
-            -d '{"Do":"merge"}' \
+            -d '{"Do":"merge","delete_branch_after_merge":true}' \
             "https://git.farh.net/api/v1/repos/cartsnitch/infra/pulls/${PR_NUM}/merge")
           MERGED=$(echo "$MERGE_RESP" | jq -r '.merged // false')
           if [ "$MERGED" != "true" ]; then