feat: translate output filename to target language

Add _translate_filename_stem() which calls the configured LLM to
translate the filename stem before building the export map. Result:
磨粉.pdf translated to English → grinding.pdf (instead of 磨粉_translated.pdf).
Falls back to original stem on any API error with zero impact on translation flow.

docutranslate/app.py

@@ -7,6 +7,7 @@ import binascii
 import json
 import logging
 import os
+import re
 import shutil
 import socket
 import tempfile
@@ -626,6 +627,47 @@ class TranslateServiceRequest(BaseModel):
     )
 
 
+async def _translate_filename_stem(
+    stem: str, to_lang: str, base_url: str | None, api_key: str, model_id: str | None
+) -> str:
+    """Translate a filename stem to target language for cleaner download filenames."""
+    try:
+        clean_base = (base_url or "").strip().rstrip("/")
+        clean_model = (model_id or "").strip()
+        if not clean_base or not clean_model:
+            return stem
+        resp = await httpx_client.post(
+            f"{clean_base}/chat/completions",
+            headers={
+                "Authorization": f"Bearer {(api_key or '').strip()}",
+                "Content-Type": "application/json",
+            },
+            json={
+                "model": clean_model,
+                "messages": [
+                    {
+                        "role": "user",
+                        "content": (
+                            f"Translate the document filename below into {to_lang}. "
+                            "Return ONLY the translated name, no file extension, no explanation, no quotes.\n"
+                            f"Filename: {stem}"
+                        ),
+                    }
+                ],
+                "max_tokens": 60,
+                "temperature": 0,
+            },
+            timeout=10,
+        )
+        resp.raise_for_status()
+        translated = resp.json()["choices"][0]["message"]["content"].strip().strip("\"'")
+        sanitized = re.sub(r"[^\w\s\-]", "", translated).strip()
+        sanitized = re.sub(r"\s+", "_", sanitized)[:50]
+        return sanitized if sanitized else stem
+    except Exception:
+        return stem
+
+
 # --- Background Task Logic ---
 async def _perform_translation(
         task_id: str,
@@ -1131,6 +1173,11 @@ async def _perform_translation(
         task_state["temp_dir"] = temp_dir
         downloadable_files = {}
         filename_stem = task_state["original_filename_stem"]
+        translated_stem = await _translate_filename_stem(
+            filename_stem, payload.to_lang,
+            getattr(payload, "base_url", None), getattr(payload, "api_key", ""),
+            getattr(payload, "model_id", None),
+        )
 
         # 检查CDN可用性
         is_cdn_available = True
@@ -1149,66 +1196,66 @@ async def _perform_translation(
         if isinstance(workflow, MDFormatsExportable):
             export_map["markdown"] = (
                 workflow.export_to_markdown,
-                f"{filename_stem}_translated.md",
+                f"{translated_stem}.md",
                 True,
             )
             export_map["markdown_zip"] = (
                 workflow.export_to_markdown_zip,
-                f"{filename_stem}_translated.zip",
+                f"{translated_stem}.zip",
                 False,
             )
         if isinstance(workflow, TXTExportable):
             export_map["txt"] = (
                 workflow.export_to_txt,
-                f"{filename_stem}_translated.txt",
+                f"{translated_stem}.txt",
                 True,
             )
         if isinstance(workflow, JsonExportable):
             export_map["json"] = (
                 workflow.export_to_json,
-                f"{filename_stem}_translated.json",
+                f"{translated_stem}.json",
                 True,
             )
         if isinstance(workflow, XlsxExportable):
             export_map["xlsx"] = (
                 workflow.export_to_xlsx,
-                f"{filename_stem}_translated.xlsx",
+                f"{translated_stem}.xlsx",
                 False,
             )
         if isinstance(workflow, CsvExportable):
             export_map["csv"] = (
                 workflow.export_to_csv,
-                f"{filename_stem}_translated.csv",
+                f"{translated_stem}.csv",
                 False,
             )
         if isinstance(workflow, DocxExportable):
             export_map["docx"] = (
                 workflow.export_to_docx,
-                f"{filename_stem}_translated.docx",
+                f"{translated_stem}.docx",
                 False,
             )
         if isinstance(workflow, SrtExportable):
             export_map["srt"] = (
                 workflow.export_to_srt,
-                f"{filename_stem}_translated.srt",
+                f"{translated_stem}.srt",
                 True,
             )
         if isinstance(workflow, EpubExportable):
             export_map["epub"] = (
                 workflow.export_to_epub,
-                f"{filename_stem}_translated.epub",
+                f"{translated_stem}.epub",
                 False,
             )
         if isinstance(workflow, AssExportable):
             export_map["ass"] = (
                 workflow.export_to_ass,
-                f"{filename_stem}_translated.ass",
+                f"{translated_stem}.ass",
                 True,
             )
         if isinstance(workflow, PPTXExportable):
             export_map["pptx"] = (
                 workflow.export_to_pptx,
-                f"{filename_stem}_translated.pptx",
+                f"{translated_stem}.pptx",
                 False,
             )
 
@@ -1235,7 +1282,7 @@ async def _perform_translation(
                 html_config = PPTX2HTMLExporterConfig(cdn=is_cdn_available)
             export_map["html"] = (
                 lambda: workflow.export_to_html(html_config),
-                f"{filename_stem}_translated.html",
+                f"{translated_stem}.html",
                 True,
             )