fix(ci): push 路径检测改用 compare 响应内每提交 files 聚合

Gitea 1.26.4 compare API 顶层 files 字段始终为空且 merge_base=null，
导致 ci_push_paths.sh 永远走保守全量构建，路径过滤形同虚设。
改为聚合 compare 响应中 commits[].files（该字段完整可用），
覆盖 squash merge（单提交）与多提交 push 区间。
已在本地用三组真实区间验证：纯后端6文件、纯前端3文件、全栈11文件。

scripts/ci/ci_push_paths.sh

@@ -30,7 +30,8 @@ echo "改动范围检测: before=${before:-<empty>} after=${after}"
 
 FILES=""
 if [ -n "$before" ] && [ "$before" != "0000000000000000000000000000000000000000" ]; then
-  # compare API: {base}...{head}
+  # Gitea 1.26.x compare API 的顶层 files 字段不填充（始终为空），
+  # 但响应里每个 commit 条目自带的 files 完整可用；聚合区间内所有提交的 files 即可。
   API_URL="${base}/repos/${repo}/compare/${before}...${after}?per_page=300"
   for attempt in 1 2 3; do
     FILES=$(curl -s --max-time 30 -H "Authorization: token ${GITHUB_TOKEN}" "$API_URL" \
@@ -38,8 +39,13 @@ if [ -n "$before" ] && [ "$before" != "0000000000000000000000000000000000000000"
 import json,sys
 try:
     d=json.load(sys.stdin)
-    for f in d.get('files', []):
-        print(f.get('filename',''))
+    files=set()
+    for c in d.get('commits', []) or []:
+        for f in (c.get('files') or []):
+            n = f.get('filename') or ''
+            if n:
+                files.add(n)
+    print('\n'.join(sorted(files)))
 except Exception:
     pass
 ")
@@ -62,11 +68,11 @@ BACKEND_COUNT=$(python3 -c "print($TOTAL - $FRONTEND_COUNT)")
 
 echo "变更文件: ${TOTAL} 个 (前端: ${FRONTEND_COUNT}, 后端/公共: ${BACKEND_COUNT})"
 
-if [ "$BACKEND_COUNT" = "0" ] && [ "$FRONTEND_COUNT" -gt "0" ]; then
+if [ "$BACKEND_COUNT" = "0" ] && [ "$FRONTEND_COUNT" -gt 0 ]; then
   echo "skip_backend=true" >> "$OUTPUT"
   echo "skip_frontend=false" >> "$OUTPUT"
   echo "✅ 纯前端改动，跳过后端镜像构建"
-elif [ "$FRONTEND_COUNT" = "0" ] && [ "$BACKEND_COUNT" -gt "0" ]; then
+elif [ "$FRONTEND_COUNT" = "0" ] && [ "$BACKEND_COUNT" -gt 0 ]; then
   echo "skip_backend=false" >> "$OUTPUT"
   echo "skip_frontend=true" >> "$OUTPUT"
   echo "🔧 纯后端改动，跳过 Web 镜像构建"