fix: make release PR creation robust against race conditions (#19)

Two fixes for intermittent release workflow failures:

- **Use `BASE_BRANCH` instead of `DEFAULT_BRANCH`** for the PR `base` field — `BASE_BRANCH` has a `:-main` fallback, preventing an empty base from causing a 404
- **Replace fixed `sleep 3` with proper polling and retry** — polls for branch readiness (up to 10 attempts) before creating the PR, then retries PR creation (up to 5 attempts) with backoff

Fixes the `Error creating PR (HTTP 404)` seen when Gitea hasn't fully indexed the `next-release` branch by the time the PR creation request fires.

🤖 Generated with [Claude Code](https://claude.ai/claude-code)

Reviewed-on: #19

.gitea/workflows/Release.yml

@@ -189,27 +189,51 @@ jobs:
           fi
 
           echo "Creating new PR..."
-          echo "Waiting for branch to be ready..."
-          sleep 3
-
-          RESPONSE=$(curl -s --retry 3 --retry-delay 3 --retry-all-errors --retry-connrefused \
-            -w "\n%{http_code}" -X POST \
+          echo "Waiting for next-release branch to be ready..."
+          for i in $(seq 1 10); do
+            BRANCH_STATUS=$(curl -s --retry 3 --retry-delay 2 --retry-connrefused \
+              -w "%{http_code}" -o /dev/null \
               -H "Authorization: token ${TOKEN}" \
-            -H "Content-Type: application/json" \
-            --data "$(jq -n \
+              "${API_URL}/branches/next-release")
+            if [ "${BRANCH_STATUS}" = "200" ]; then
+              echo "Branch ready after ${i} attempt(s)"
+              break
+            fi
+            if [ "${i}" = "10" ]; then
+              echo "Branch next-release not found after 10 attempts, giving up"
+              exit 1
+            fi
+            echo "Branch not ready yet (attempt ${i}/10), waiting..."
+            sleep 3
+          done
+
+          PR_DATA=$(jq -n \
             --arg title "${TITLE}" \
             --arg body "${DESCRIPTION}" \
             --arg head "next-release" \
-              --arg base "${DEFAULT_BRANCH}" \
-              '{title: $title, body: $body, head: $head, base: $base}')" \
+            --arg base "${BASE_BRANCH}" \
+            '{title: $title, body: $body, head: $head, base: $base}')
+
+          for i in $(seq 1 5); do
+            RESPONSE=$(curl -s --retry 3 --retry-delay 2 --retry-connrefused \
+              -w "\n%{http_code}" -X POST \
+              -H "Authorization: token ${TOKEN}" \
+              -H "Content-Type: application/json" \
+              --data "${PR_DATA}" \
               "${API_URL}/pulls")
             HTTP_CODE=$(echo "${RESPONSE}" | tail -1)
             BODY=$(echo "${RESPONSE}" | sed '$d')
-          if [ "${HTTP_CODE}" -ge 400 ]; then
-            echo "Error creating PR (HTTP ${HTTP_CODE}): ${BODY}"
+            if [ "${HTTP_CODE}" -lt 400 ]; then
+              echo "PR created successfully"
+              break
+            fi
+            if [ "${i}" = "5" ]; then
+              echo "Error creating PR after 5 attempts (HTTP ${HTTP_CODE}): ${BODY}"
               exit 1
             fi
-          echo "PR created successfully"
+            echo "PR creation attempt ${i}/5 failed (HTTP ${HTTP_CODE}), retrying..."
+            sleep 3
+          done
 
   create-release:
     name: Create Release