feat(translation): quality pipeline overhaul + new features (audit 2026-08-29)

Translation quality & format preservation:
- Word: merge adjacent same-format runs into one unit (sentence-level
  coherence like inline-tag handling); translate comments/balloons;
  dedupe textbox collection (was translated twice); RTL no longer
  overrides center/justify alignment; CJK/Arabic font hints (eastAsia/cs)
- PPTX: chart translations now actually reach the output file
  (ChartPart.blob is read-only — rewrite chart XML in the saved ZIP);
  CJK typeface hints (a:ea)
- Excel: sheet renames no longer break references — rewrite cell
  formulas (3D/quoted), defined names, data validations, cond. formats
- PDF: bold/italic honored (hebo/heit/hebi); table cells never merge;
  unchanged blocks left untouched (typography preserved, fixes duplicate
  hyperlinks); attempted/changed stats + route gate now cover PDF;
  CJK font paths; scanned PDFs via Mistral OCR (detection + admin settings)

Features:
- formality param (formal/informal) + automatic regional-variant prompts
- output_mode=bilingual docx (source above translation)
- per-user translation memory on Redis (falls back to LRU), context-hashed
- QA report + 0-100 confidence score in job status; L0 on by default
- OpenAI-compatible providers: whole chunk in ONE numbered-JSON request
  (~15x fewer calls) with per-item fallback; base prompt always present
  (custom prompt no longer replaces translation instructions)

Infra & marketing alignment:
- plan-based engine gating + vision gating (closes paid-engine leak);
  /providers/available filtered per plan; 107 languages exposed
- zh-CN/zh-TW validation fixed; libmagic disabled on Windows (native crash)
- admin: Mistral OCR settings + engine status dashboard; httpx<0.28 pin
  (TestClient breakage); Prometheus test fixture fixed
- marketing docs aligned with code (PDF+OCR, retention, engines, pricing)
- security: .env.ionos/.env.production/provider_settings.json removed

Tests: 1173 passed / 0 failed (6 network tests deselected: free Google
endpoint temporarily blocked from this machine)

services/providers/deepseek_provider.py

@@ -39,16 +39,10 @@ Rules:
 
 
 def _get_language_name(code: str) -> str:
-    language_names = {
-        "en": "English", "fr": "French", "es": "Spanish", "de": "German",
-        "it": "Italian", "pt": "Portuguese", "nl": "Dutch", "ru": "Russian",
-        "zh": "Chinese", "ja": "Japanese", "ko": "Korean", "ar": "Arabic",
-        "hi": "Hindi", "tr": "Turkish", "pl": "Polish", "vi": "Vietnamese",
-        "th": "Thai", "uk": "Ukrainian", "cs": "Czech", "sv": "Swedish",
-        "ro": "Romanian", "hu": "Hungarian", "el": "Greek", "he": "Hebrew",
-    }
-    return language_names.get(code.split("-")[0].lower(), code)
+    """Convert language code to full name (all supported languages)."""
+    from core.languages import language_name
 
+    return language_name(code)
 
 class DeepSeekProviderError(Exception):
     def __init__(self, code: str, message: str, details: Optional[Dict[str, Any]] = None):
@@ -161,9 +155,19 @@ class DeepSeekTranslationProvider(TranslationProvider):
         source_lang_name = _get_language_name(source_language)
         target_lang_name = _get_language_name(target_language)
         custom_prompt = request.metadata.get("custom_prompt") if request.metadata else None
-        system_prompt = custom_prompt or DEFAULT_TRANSLATION_PROMPT.format(
+        _base_prompt = DEFAULT_TRANSLATION_PROMPT.format(
             source_lang=source_lang_name, target_lang=target_lang_name
         )
+        # Base translation instructions always present; the custom prompt
+        # (glossary/tone/context) is appended, never a replacement.
+        if custom_prompt and custom_prompt.strip():
+            system_prompt = (
+                _base_prompt
+                + "\n\nADDITIONAL CONTEXT AND INSTRUCTIONS:\n"
+                + custom_prompt.strip()
+            )
+        else:
+            system_prompt = _base_prompt
 
         last_error = None
         for attempt in range(self._max_retries + 1):