fix: 所有问题都建 Issue、合并失败重试、push 与 PR 事件分流

1. 所有问题都建 Issue（原先只有阻断级才建）
   建 Issue 的判据从 blocking 改为 findings：任何等级的问题都创建/更新 Issue，
   等级体现在标题 [OCR][medium] 与标签 medium 上，阻断项在正文标注
   「（阻断合并）」。低等级问题不再丢失，也不会挡住合并。
   无任何发现时才关闭该分支的 Issue。

2. 合并失败自动重试
   Gitea 在算完 PR 可合并性之前会返回 405 Please try again later，
   原先直接放弃，晋级随机失败。现在对 405/409/5xx 按指数退避重试 4 次；
   权限不足、真实冲突等永久失败立即放弃并往 PR 留言说明。
   PR 已被合并（405 already merged）视为成功，幂等收尾。

3. push 与 PR 事件分流（本次新发现的 bug）
   合并路径原先按「是否找到关联 PR」判断，于是一个残留的 test→prd PR
   会让后续 push 被当成 PR 事件，走错分支并跳过晋级，日志还会给出
   「PR targets prd, which is not a checked branch」这种与实际不符的原因。
   现在按事件类型决定：trigger 以 pull_request 开头才走合并 PR 路径，
   push/manual 一律走晋级分支。关联 PR 仅用于评论归属。

验证：
- 直接 push test → 建出 issue #4（[OCR][medium]，标签 code-review,medium）
- 合并重试与幂等分支的判定表全部通过
- 事件分流判定表：push/manual → promote，pull_request.* → merge PR

app/lib/gitea.js

@@ -345,6 +345,44 @@ export class GiteaClient {
   }
 }
 
+/**
+ * Gitea answers 405 "Please try again later" while it is still computing a pull
+ * request's mergeability, and 409 for transient lock contention. Both clear up
+ * on their own, so a merge is retried on those before giving up.
+ */
+export function isTransientMergeError(err) {
+  if (!err) return false;
+  const msg = String(err.message || "");
+  // Already merged is a final state, not something to retry.
+  if (/already merged/i.test(msg)) return false;
+  if (err.status === 405 || err.status === 409) return true;
+  if (err.status === 500 || err.status === 502 || err.status === 503 || err.status === 504) return true;
+  return /try again later|please try again|mergeability|not ready/i.test(msg);
+}
+
+/** True when a merge failed only because the pull request is already merged. */
+export function isAlreadyMerged(err) {
+  return /already merged/i.test(String(err?.message || ""));
+}
+
+/** Retry `fn` while it throws a transient error, with exponential backoff. */
+export async function retryTransient(fn, { attempts = 4, baseDelayMs = 1500, onRetry } = {}) {
+  let delay = baseDelayMs;
+  let lastErr;
+  for (let attempt = 1; attempt <= attempts; attempt += 1) {
+    try {
+      return await fn();
+    } catch (err) {
+      lastErr = err;
+      if (!isTransientMergeError(err) || attempt === attempts) throw err;
+      onRetry?.(attempt, err);
+      await new Promise((r) => setTimeout(r, delay));
+      delay = Math.min(delay * 2, 12000);
+    }
+  }
+  throw lastErr;
+}
+
 /** Derive the repository web URL from an API base URL. */
 export function webBaseUrl(apiBaseUrl) {
   return normalizeBaseUrl(apiBaseUrl);