fix(cover): add plan.config.cover_candidates fallback in cover generation endpoint

Problem:
The cover generation endpoint (POST /generation/generate-cover) only
checks GenerationTask.cover_url through 3 fallback paths (A: direct
task_id, B: source_edit_plan_id, C: user+template). However, the
worker writes cover candidate frames to plan.config['cover_candidates']
during preview rendering, and this field was never read by the endpoint.

When GenerationTask.cover_url was empty (e.g., due to cover frame
extraction key mismatch or old data), the endpoint returned 400 even
though cover_candidates existed in plan.config.

Fix:
Add step D fallback that reads plan.config.cover_candidates[0] before
returning 400. This covers cases where:
- The worker wrote cover_candidates but GenerationTask.cover_url was
  not set (historical data, key mismatch)
- The cover frames were extracted during rendering but the task-level
  cover_url write was skipped

Also update the 400 warning log to indicate which steps were checked.

apps/api/app/api/routes/generation_cover.py

@@ -310,6 +310,22 @@ def generate_cover(
                     exc_info=True,
                 )
 
+        # 步骤 D：从 plan.config.cover_candidates 读取（Worker 渲染时写入）
+        if not cover_url_from_task:
+            _candidates = (plan.config or {}).get("cover_candidates") or []
+            if isinstance(_candidates, list) and _candidates:
+                _first = _candidates[0]
+                if isinstance(_first, dict):
+                    cover_url_from_task = (
+                        _first.get("image_url") or _first.get("url") or ""
+                    )
+                    if cover_url_from_task:
+                        logger.info(
+                            "[封面生成] 统一管道封面(步骤D-cover_candidates): plan_id=%s url=%s",
+                            plan_id,
+                            cover_url_from_task[:80],
+                        )
+
         if cover_url_from_task:
             # 标题已在预览视频渲染时烧录（ASS字幕），封面帧自然包含标题
             cover_data = {
@@ -325,7 +341,7 @@ def generate_cover(
             return GenerateCoverResponse(plan_id=plan_id, cover=cover_data)
 
         logger.warning(
-            "[封面生成] 统一管道未找到 cover_url: plan_id=%s",
+            "[封面生成] 统一管道未找到 cover_url (A/B/C/D均未命中): plan_id=%s",
             plan_id,
         )
         # ai_frame/ai_regenerate 类型必须从渲染管道获取，不再回退到 AI 服务