fix: 标题默认位置 fallback 改为 bottom，与前端 DEFAULT_TITLE_SETTINGS 对齐

- position_to_ass_alignment() fallback 从 8(top) 改为 2(bottom)
- build_ass_content() title 默认 position 从 'top' 改为 'bottom'
- subtitle_generator.generate_ass_from_timeline() title 默认 position 同步
- 更新 6 个测试文件中 fallback 断言（8→2）
- 新增 TestDefaultPositionBottom 测试类（3 个测试）
- 351 个标题/字幕相关测试全通过

apps/worker/video_processing/subtitle_generator.py

@@ -213,7 +213,7 @@ def generate_ass_from_timeline(
             t_shadow.get("offset_x", 2) if t_shadow.get("enabled", False) else 0,
             t_shadow.get("offset_y", 2) if t_shadow.get("enabled", False) else 0,
         )
-        t_alignment = position_to_ass_alignment(title_cfg.get("position", "top"))
+        t_alignment = position_to_ass_alignment(title_cfg.get("position", "bottom"))
 
         title_style_line = build_ass_style(
             "TitleStyle",

packages/domain/ass_subtitle_builder.py

@@ -81,14 +81,14 @@ def position_to_ass_alignment(position: str) -> int:
         position: 位置字符串 top/center/bottom
 
     Returns:
-        ASS 对齐编号，默认 8（顶部居中）
+        ASS 对齐编号，默认 2（底部居中，与前端 DEFAULT_TITLE_SETTINGS.position="bottom" 对齐）
     """
     mapping = {
         "top": 8,
         "center": 5,
         "bottom": 2,
     }
-    return mapping.get(position, 8)
+    return mapping.get(position, 2)
 
 
 # ── Style 行构建 ──────────────────────────────────────────────────────────────
@@ -226,7 +226,6 @@ def _wrap_title_text(
 
     # 换行计算使用原始 font_size，与 CSS 预览一致；1.35x 补偿仅用于 ASS Fontsize 渲染
 
-
     # 先按已有 \N 分段，每段独立自动换行，最后用 \N 拼回
     segments = text.split("\\N")
     wrapped_segments: list[str] = []
@@ -386,8 +385,8 @@ def build_ass_content(
         # position → alignment 三档逻辑，现有输出保持一字节不变。
         title_pos = _parse_title_position(title_config, video_width, video_height)
 
-        title_alignment = 5 if title_pos is not None else position_to_ass_alignment(
-            title_config.get("position", "top")
+        title_alignment = (
+            5 if title_pos is not None else position_to_ass_alignment(title_config.get("position", "bottom"))
         )
 
         styles.append(

tests/unit/domain/test_ass_subtitle_builder.py

@@ -83,11 +83,11 @@ class TestPositionToAssAlignment:
     def test_bottom(self):
         assert position_to_ass_alignment("bottom") == 2
 
-    def test_unknown_defaults_top(self):
-        assert position_to_ass_alignment("unknown") == 8
+    def test_unknown_defaults_bottom(self):
+        assert position_to_ass_alignment("unknown") == 2
 
-    def test_empty_defaults_top(self):
-        assert position_to_ass_alignment("") == 8
+    def test_empty_defaults_bottom(self):
+        assert position_to_ass_alignment("") == 2
 
 
 # ============================================================
@@ -581,6 +581,7 @@ class TestConstants:
         assert isinstance(TITLE_MARGIN_BOTTOM, int)
         assert isinstance(TITLE_MARGIN_SIDE, int)
 
+
 # ============================================================
 # _wrap_title_text 换行逻辑验证
 # ============================================================

tests/unit/test_ass_subtitle_builder.py

@@ -65,11 +65,11 @@ class TestPositionToAssAlignment:
     def test_bottom(self):
         assert position_to_ass_alignment("bottom") == 2
 
-    def test_unknown_default_top(self):
-        assert position_to_ass_alignment("unknown") == 8
+    def test_unknown_default_bottom(self):
+        assert position_to_ass_alignment("unknown") == 2
 
-    def test_empty_default_top(self):
-        assert position_to_ass_alignment("") == 8
+    def test_empty_default_bottom(self):
+        assert position_to_ass_alignment("") == 2
 
 
 # ── Style 行构建 ─────────────────────────────────────────────────────────────
@@ -747,3 +747,45 @@ class TestTitleFreePosition:
             line for line in content.splitlines() if line.startswith("Dialogue:") and "SubtitleStyle" in line
         ][0]
         assert "\\pos(" not in sub_dialogue
+
+
+class TestDefaultPositionBottom:
+    """默认 position 应为 bottom（alignment=2），与前端 DEFAULT_TITLE_SETTINGS 对齐。"""
+
+    def _base_kwargs(self):
+        return dict(
+            video_width=1080,
+            video_height=1920,
+            video_duration=10.0,
+            title_text="测试标题",
+        )
+
+    def test_no_position_defaults_to_bottom_alignment(self):
+        """不传 position 时，Alignment 应为 2（bottom）。"""
+        content = build_ass_content(
+            **self._base_kwargs(),
+            title_config={"size": 36},
+        )
+        style_line = [line for line in content.splitlines() if line.startswith("Style: TitleStyle")][0]
+        fields = [f.strip() for f in style_line.split(",")]
+        assert fields[18] == "2", f"Expected alignment 2 (bottom), got {fields[18]}"
+
+    def test_no_position_no_coords_defaults_to_bottom(self):
+        """不传 position 也不传坐标时，走 bottom 三档逻辑。"""
+        content = build_ass_content(
+            **self._base_kwargs(),
+            title_config={},
+        )
+        style_line = [line for line in content.splitlines() if line.startswith("Style: TitleStyle")][0]
+        fields = [f.strip() for f in style_line.split(",")]
+        assert fields[18] == "2"
+
+    def test_explicit_top_still_works(self):
+        """显式传 position='top' 仍然得到 alignment=8。"""
+        content = build_ass_content(
+            **self._base_kwargs(),
+            title_config={"position": "top", "size": 36},
+        )
+        style_line = [line for line in content.splitlines() if line.startswith("Style: TitleStyle")][0]
+        fields = [f.strip() for f in style_line.split(",")]
+        assert fields[18] == "8"

tests/unit/test_render_subtitles.py

@@ -67,11 +67,11 @@ class TestPositionToAssAlignment:
     def test_bottom(self):
         assert _position_to_ass_alignment("bottom") == 2
 
-    def test_unknown_returns_top_default(self):
-        assert _position_to_ass_alignment("unknown") == 8
-        assert _position_to_ass_alignment("") == 8
-        assert _position_to_ass_alignment("left") == 8
-        assert _position_to_ass_alignment(None) == 8
+    def test_unknown_returns_bottom_default(self):
+        assert _position_to_ass_alignment("unknown") == 2
+        assert _position_to_ass_alignment("") == 2
+        assert _position_to_ass_alignment("left") == 2
+        assert _position_to_ass_alignment(None) == 2
 
 
 class TestBuildAssStyle:

tests/unit/test_render_subtitles_pure.py

@@ -66,15 +66,15 @@ class TestPositionToAssAlignment:
         """center → 居中(5)."""
         assert _position_to_ass_alignment("center") == 5
 
-    def test_unknown_defaults_to_top(self):
-        """未知位置默认顶部(8)."""
-        assert _position_to_ass_alignment("unknown") == 8
-        assert _position_to_ass_alignment("top_left") == 8
-        assert _position_to_ass_alignment("bottom_right") == 8
+    def test_unknown_defaults_to_bottom(self):
+        """未知位置默认底部(2)."""
+        assert _position_to_ass_alignment("unknown") == 2
+        assert _position_to_ass_alignment("top_left") == 2
+        assert _position_to_ass_alignment("bottom_right") == 2
 
-    def test_empty_string_defaults_to_top(self):
-        """空字符串默认顶部."""
-        assert _position_to_ass_alignment("") == 8
+    def test_empty_string_defaults_to_bottom(self):
+        """空字符串默认底部."""
+        assert _position_to_ass_alignment("") == 2
 
 
 class TestBuildAssStyle: