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_translate_endpoint.py

@@ -593,7 +593,11 @@ class TestOptionalParameters:
         assert response.status_code == 202
 
     def test_accepts_mode_llm(self, authenticated_client):
-        """Accepts mode='llm'"""
+        """mode='llm' maps to the openrouter engine — Pro and above only.
+
+        The default test user is on the Free plan, so the plan-based engine
+        gate must refuse it (403) instead of silently running a paid engine.
+        """
         excel_content = create_valid_excel()
         response = authenticated_client.post(
             TRANSLATE_URL,
@@ -606,7 +610,8 @@ class TestOptionalParameters:
             },
             data={"target_lang": "fr", "mode": "llm"},
         )
-        assert response.status_code == 202
+        assert response.status_code == 403
+        assert "PRO_FEATURE_REQUIRED" in str(response.json())
 
     def test_accepts_webhook_url(self, authenticated_client):
         """Accepts webhook_url parameter"""
@@ -875,7 +880,11 @@ class TestTranslateImagesParameter:
     """Test translate_images parameter in POST /api/v1/translate"""
 
     def test_accepts_translate_images_parameter(self, authenticated_client):
-        """Endpoint accepts translate_images form parameter"""
+        """translate_images is a Pro+ feature — Free plan gets 403.
+
+        The parameter is accepted by the schema; the plan gate refuses it
+        for the default (Free) test user.
+        """
         excel_content = create_valid_excel()
         response = authenticated_client.post(
             TRANSLATE_URL,
@@ -888,4 +897,5 @@ class TestTranslateImagesParameter:
             },
             data={"target_lang": "fr", "translate_images": "true"},
         )
-        assert response.status_code == 202
+        assert response.status_code == 403
+        assert "PRO_FEATURE_REQUIRED" in str(response.json())