feat: 字幕质量提升——长时寒暄幻觉清洗 + 专名/成人隐语不直译

两部分同属字幕质量优化，共用 llm.py 翻译链路：

1) 长时寒暄幻觉词清洗（nodes/subtitle_cleanup.py）
   对展示时长超过阈值（默认 15s，实测 30s 幻觉占位 vs 2s 真实词的分界）
   且文本含收尾/开场寒暄（晚安、感谢观看等）的字幕，文本替换为 '-'，
   由后续过滤流程移除；短时寒暄（如剧情中真实互道晚安）保留不误删。
    写文件前调用 。

2) 专名/隐语不直译（nodes/proper_nouns.py）
   片假名专名（人名/品牌/角色）与成人语境隐语是 LLM 误译重灾区：
   - ジンゴ 被误译成'芒果'、カンタくん 被保留日文而非音译；
   - マンゴー/バナナ/リンゴ/金玉/おまんこ/ちんちん 等在色情语境中
     是生殖器官代称，字面直译严重错译。
   整理三张规则表（专名/拟声词/成人隐语，用户提供隐语表），
    检测原文命中后动态注入翻译提示词，
   让 LLM 按正确语义处理。文档见 docs/proper_nouns.md。

测试（红→绿）：
- tests/test_hallucination_mask.py：长时掩码/短时保留/非寒暄保留/阈值边界
- tests/test_proper_nouns.py：专名命中/拟声词/成人隐语/真实数据集成
- 真实 LLM 集成测试（完整 1440 行翻译 + 清洗 + 专名注入）通过

nodes/subtitle_cleanup.py

@@ -0,0 +1,85 @@
+"""Subtitle cleanup: mask long-duration closing/greeting hallucinations.
+
+Background (real run 20260905115050): after fixing the timing alignment,
+subtitles still contain "closing/greeting hallucination words" - fixed
+phrases like 'wan an / gan xie guan kan / gan xie nin de guan kan'
+(good night / thanks for watching) that the ASR/LLM repeatedly emits on
+empty segments, filling a full 30s block, unrelated to video content.
+Some 2s 'good night' might be real dialogue, so it must be kept.
+
+Plan (confirmed by user): after translation, mask subtitle entries whose
+*display duration* exceeds a threshold AND whose text contains a greeting
+hallucination token - replace the text with '-' so the downstream SRT/filter
+pipeline drops it. The duration threshold protects short real greetings.
+
+Threshold is derived from real run data: 30s hallucinations vs 2s real words,
+a clear gap; default 15s (>=15s masks, <15s keeps).
+
+Pure functions, unit-testable (tests/test_hallucination_mask.py).
+"""
+
+from __future__ import annotations
+
+import re
+
+# Greeting/closing hallucination tokens that LLM repeats on empty/end segments.
+HALLUCINATION_TOKENS = (
+    "谢谢观看", "感谢观看", "感谢收看", "谢谢收看", "感谢您的观看", "感谢您的收看",
+    "晚安", "下次再见", "再会", "敬请期待", "感谢您的光临", "欢迎光临",
+    "再见", "多谢观看", "观看愉快",
+)
+
+# Display-duration threshold (seconds): only mask entries longer than this.
+DEFAULT_THRESHOLD_SECONDS = 15.0
+
+_SRT_BLOCK = re.compile(
+    r"(\d{2}:\d{2}:\d{2},\d{3})\s*-->\s*(\d{2}:\d{2}:\d{2},\d{3})\s*\n(.*?)(?=\n\s*\d+\s*\n|\Z)",
+    re.DOTALL,
+)
+
+
+def mask_hallucination_text(
+    entries: list[dict],
+    threshold_seconds: float = DEFAULT_THRESHOLD_SECONDS,
+) -> list[dict]:
+    """Return a new list where long-duration greeting entries have text='-'.
+
+    duration = end - start. Only entries whose duration >= threshold AND text
+    contains any HALLUCINATION_TOKENS are masked. Input list is not mutated.
+    """
+    cleaned = []
+    for entry in entries:
+        duration = entry.get("end", 0.0) - entry.get("start", 0.0)
+        text = entry.get("text", "")
+        if duration >= threshold_seconds and any(t in text for t in HALLUCINATION_TOKENS):
+            entry = dict(entry, text="-")
+        cleaned.append(entry)
+    return cleaned
+
+
+def _ts_to_seconds(ts: str) -> float:
+    """Convert an SRT timestamp HH:MM:SS,mmm to seconds (float)."""
+    hours, minutes, rest = ts.split(":")
+    seconds, millis = rest.split(",")
+    return int(hours) * 3600 + int(minutes) * 60 + int(seconds) + int(millis) / 1000
+
+
+def clean_srt_text(
+    srt_text: str,
+    threshold_seconds: float = DEFAULT_THRESHOLD_SECONDS,
+) -> str:
+    """Mask long-duration greeting hallucinations in an SRT string.
+
+    Parses each cue's start/end/text, applies mask_hallucination_text, and
+    rewrites the block keeping the original time line when not masked.
+    """
+    def _replace(match) -> str:
+        start = _ts_to_seconds(match.group(1))
+        end = _ts_to_seconds(match.group(2))
+        text = match.group(3).strip()
+        entry = {"start": start, "end": end, "text": text}
+        cleaned = mask_hallucination_text([entry], threshold_seconds)
+        new_text = cleaned[0]["text"]
+        return f"{match.group(1)} --> {match.group(2)}\n{new_text}"
+
+    return _SRT_BLOCK.sub(_replace, srt_text)