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)

tests/test_providers/test_openai_provider.py

@@ -97,11 +97,24 @@ class TestHelperFunctions:
         assert "translator" in prompt.lower()
 
     def test_build_system_prompt_custom(self):
-        """Test custom system prompt."""
+        """A custom prompt AUGMENTS the base translation instructions.
+
+        The base prompt is always present — a glossary-only custom prompt
+        used to produce a system prompt with no translation instruction
+        at all (fixed 2026-08-29).
+        """
         custom = "Translate this text formally for business context."
         prompt = _build_system_prompt("English", "French", custom)
 
-        assert prompt == custom
+        # Base translation instructions survive
+        assert "English" in prompt
+        assert "French" in prompt
+        assert "translator" in prompt.lower()
+        # Custom content is appended, not replacing
+        assert custom in prompt
+        assert "ADDITIONAL CONTEXT AND INSTRUCTIONS" in prompt
+        # The base part comes first
+        assert prompt.index("French") < prompt.index(custom)
 
 
 class TestOpenAITranslationProvider: