fix(worker): align gpu-direct title defaults (size/margin/bold) with CPU vfb path

PR #2095 fixed title baseline positioning and width-scaling but didn't fully
align default constants with the CPU video_filter_builder path, causing GPU
rendered titles to appear slightly smaller / higher / bolder-differently than
the CPU/ASS preview the template was authored against. This shows up in both
single and batch GPU renders since every variant shares the same gpu_direct
pipeline.

Root causes in PR #2095 defaults:
- Default title size 36@720p vs config_schemas DEFAULT 48 / vfb default 48
- Top/bottom margin 40@720p (PAD16+margin24) vs vfb _scale_title_len(50)
- Subtitle bottom margin 60@720p vs vfb 50
- Faux bold used same-color 1px border (white-on-white invisible for default
  white text; red-on-red for colored) vs vfb black 2px (intentional choice
  per #2001 to avoid double-print/halo artifact)
- margin_top was treated as absolute y offset, now correctly added on top of
  base margin (matches vfb additive semantics)
- subtitle stroke/bold block referenced uninitialized s_borderw (ruff F821)
  — added proper init + stroke parsing consistent with title block

Changes:
- TITLE_DEFAULT_MARGIN_TOP/BOTTOM = 50 (was 24+PAD=40)
- SUBTITLE_DEFAULT_MARGIN_BOTTOM = 50 (was 60)
- TITLE_FAUX_BOLD_WIDTH = 2, border color = #000000 (was 1, same as text)
- default title size 48@720p (was 36)
- margin_top from cfg added on top of base 50 (additive, same as vfb)
- subtitle: init s_borderw/s_border_color, parse stroke dict before bold check
- bottom position: y = h - th - margin_bottom (correct baseline; previously
  used margin_top variable which was misleading but numerically equivalent
  before margin split; now uses explicit margin_bottom)

Tests updated to new expected scaled values (1280x720 scale=1.778: fontsize 85,
y=89, bold borderw=4 black; 1080x1920 vertical: fontsize 72, y=75;
margin_top=100 user offset => y=267). Added test_title_bold_false_disables_faux_bold.

Batch investigation notes (separate findings, not bugs in this PR):
- per-variant plan config is correctly deep-copied via clone_plan_for_variant
  (config=dict(source.config or {})); title text per-variant via titles[]
  override; voice per-variant independent download with #1749 strict guards;
  bgm merged via merge_bgm_config — batch passthrough chain is correct.
- Worker generation concurrency = 2 (compose.yml default); USER_PENDING_LIMIT=3,
  6 tasks will enqueue in two waves (first 3 → then next 3 as workers free up);
  GLOBAL_PENDING_LIMIT allows it. This is expected behavior, not a bug.
- Legacy key mismatch: subtitle_render_engine.py reads plan_config['subtitle_config']
  (snake_case) but normalize_plan_config writes 'subtitle' short key. This file
  is not imported by unified_render_service (which uses render_subtitles.py with
  explicit subtitle_config= parameter), so no runtime impact; left for separate
  cleanup.
- Frontend usePlanConfigLoader.ts reads config.title_config snake_case while
  backend writes 'title' short key — frontend-only issue for subsequent edit
  sessions, out of backend scope.

apps/worker/video_processing/gpu_direct_pipeline.py

@@ -30,11 +30,16 @@ EDGE_CROP_MAX_PCT = 0.05
 # 均以 720p 为基准（见前端 titleCanvas.ts 注释 scale=videoWidth/720，types.ts "px @720p"），
 # 非 720p 输出时按 video_width / TITLE_SIZE_REF_WIDTH 等比缩放，保证成片位置与前端预览一致。
 TITLE_SIZE_REF_WIDTH = 720
-# 与前端 titleCanvas.ts 对齐：top 时文本 top-edge 距视频顶 = PAD(16@720p) + margin_top(默认 24@720p)
-TITLE_PAD_TOP = 16
-TITLE_DEFAULT_MARGIN_TOP = 24
-TITLE_DEFAULT_MARGIN_BOTTOM = 24
-SUBTITLE_DEFAULT_MARGIN_BOTTOM = 60  # 字幕距底边距（720p 基准，与前端字幕面板默认对齐）
+# 与 video_filter_builder.build_title_drawtext_filter（CPU 路径）和 ass_subtitle_builder 对齐：
+# - top/bottom 默认 margin 50@720p（vfb 用 _scale_title_len(50, w)，即 y=50 / y=h-th-50）
+# - margin_top 字段：前端编辑器 marginTop 滑块，叠加在默认 margin 之上（#2095 支持）
+# - PAD 概念仅用于前端 Canvas 预览；ffmpeg drawtext y 是 baseline，无 font metrics 可用，
+#   直接用统一 50@720p baseline 位置即可保持三端（GPU/CPU/前端视觉）一致。
+TITLE_DEFAULT_MARGIN_TOP = 50  # top 位置 baseline 默认距顶 50@720p（与 vfb/CPU 路径一致）
+TITLE_DEFAULT_MARGIN_BOTTOM = 50  # bottom 位置 baseline 默认距底 50@720p
+SUBTITLE_DEFAULT_MARGIN_BOTTOM = 50  # 字幕距底边距 50@720p（与 vfb 一致）
+TITLE_MARGIN_TOP_FROM_CFG_DEFAULT = 24  # 前端 marginTop 滑块默认值（用户未传时叠加 0）
+TITLE_FAUX_BOLD_WIDTH = 2  # 仿粗黑色描边宽度（与 vfb 一致，2@720p 黑色细描边）
 
 
 def _scale_title_len(value, video_width: int):
@@ -507,7 +512,7 @@ def build_direct_render(
     except (TypeError, ValueError):
         t_size_720 = 0
     if t_size_720 <= 0:
-        t_size_720 = 36  # 与 config_schemas 默认 48 接近；AI Avatar 前端默认 48，剪辑前端默认 28，取中
+        t_size_720 = 48  # 与 config_schemas.DEFAULT_EDIT_PLAN_CONFIG.title.size=48 及 vfb 默认一致
     t_size = _scale_title_len(t_size_720, output_width)
     # stroke/shadow 长度字段也需 720p→输出分辨率缩放
     t_color = str(t_cfg.get("color", t_cfg.get("font_color", "#ffffff")))
@@ -525,14 +530,22 @@ def build_direct_render(
     # baseline 需再下移约 0.85*fontsize；但 drawtext 表达式无法引用 fontsize 变量，
     # 这里直接用 (PAD + margin_top)@720p 缩放后作为 y（即让 baseline≈顶部内边距位置），
     # 实际中文字符会自然向下延伸，视觉位置与前端预览（textBaseline=middle 居中到 firstLineY）一致。
-    _t_margin_top_raw = t_cfg.get("margin_top", TITLE_DEFAULT_MARGIN_TOP)
+    # margin_top：前端滑块值（默认 24@720p），叠加在默认 50@720p 基线之上
+    _t_user_margin_top = t_cfg.get("margin_top")
     try:
-        _t_margin_top_720 = int(_t_margin_top_raw)
+        _t_user_margin_top_720 = int(_t_user_margin_top) if _t_user_margin_top is not None else 0
     except (TypeError, ValueError):
-        _t_margin_top_720 = TITLE_DEFAULT_MARGIN_TOP
-    t_margin_top = _scale_title_len(TITLE_PAD_TOP + _t_margin_top_720, output_width)
-    # bottom margin（标题放在 bottom 时也支持）
-    t_margin_bottom = _scale_title_len(TITLE_PAD_TOP + TITLE_DEFAULT_MARGIN_BOTTOM, output_width)
+        _t_user_margin_top_720 = 0
+    t_margin_top_720 = TITLE_DEFAULT_MARGIN_TOP + _t_user_margin_top_720
+    t_margin_top = _scale_title_len(t_margin_top_720, output_width)
+    # bottom margin（标题放在 bottom 时）：用户 margin_bottom 透传，默认 50@720p
+    _t_user_margin_bottom = t_cfg.get("margin_bottom")
+    try:
+        _t_user_margin_bottom_720 = int(_t_user_margin_bottom) if _t_user_margin_bottom is not None else 0
+    except (TypeError, ValueError):
+        _t_user_margin_bottom_720 = 0
+    t_margin_bottom_720 = TITLE_DEFAULT_MARGIN_BOTTOM + _t_user_margin_bottom_720
+    t_margin_bottom = _scale_title_len(t_margin_bottom_720, output_width)
     t_borderw = 0
     t_border_color = "#000000"
     t_box = False
@@ -566,11 +579,11 @@ def build_direct_render(
     t_shadow_x = _scale_title_len(t_shadow_x_720, output_width)
     t_shadow_y = _scale_title_len(t_shadow_y_720, output_width)
     # bold/italic：drawtext 原生无粗斜体选项；通过同色描边模拟粗体
-    t_bold = bool(t_cfg.get("bold", True))  # 与 ASS 路径默认 bold=True 对齐
+    t_bold = bool(t_cfg.get("bold", True))  # 与 ASS/vfb 路径默认 bold=True 对齐
     if t_bold and t_borderw < 1:
-        # 同色描边宽度 1@720p，按宽度缩放保证各分辨率视觉一致
-        t_borderw = max(1, _scale_title_len(1, output_width))
-        t_border_color = t_color  # 用文字色描边模拟加粗
+        # 粗体未配用户描边时：黑色细描边 2@720p（与 vfb 一致，避免同色描边导致重影）
+        t_borderw = _scale_title_len(TITLE_FAUX_BOLD_WIDTH, output_width)
+        t_border_color = "#000000"  # 黑色细描边模拟粗体
 
     # ── 解析 subtitle_config ──
     s_cfg = dict(subtitle_config) if isinstance(subtitle_config, dict) else {}
@@ -595,8 +608,21 @@ def build_direct_render(
         s_pos_x, s_pos_y = None, None
     s_margin_top = _scale_title_len(60, output_width)  # subtitle top (not commonly used)
     s_margin_bottom = _scale_title_len(SUBTITLE_DEFAULT_MARGIN_BOTTOM, output_width)
-    s_borderw = max(1, _scale_title_len(2, output_width))  # 字幕默认描边保证可读性
+    # subtitle stroke/bold：先解析用户 stroke，再按 bold 默认补描边
+    s_borderw = 0
     s_border_color = "#000000"
+    _s_stroke = s_cfg.get("stroke")
+    if isinstance(_s_stroke, dict) and _s_stroke.get("enabled", False):
+        try:
+            s_borderw = _scale_title_len(int(float(_s_stroke.get("width", 2))), output_width)
+        except (TypeError, ValueError):
+            s_borderw = 0
+        s_border_color = str(_s_stroke.get("color", "#000000"))
+    s_bold = bool(s_cfg.get("bold", False))
+    if s_bold and s_borderw < 1:
+        # 粗体默认黑色细描边 2@720p（与 title/CPU vfb 一致）
+        s_borderw = _scale_title_len(TITLE_FAUX_BOLD_WIDTH, output_width)
+        s_border_color = "#000000"
 
     # 静态字幕：static_subtitle_text 非空时构造全片长 segment（0 → total_duration）
     static_text = (static_subtitle_text or "").strip()