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)

.env.example

@@ -71,6 +71,16 @@ OPENAI_HEALTH_CHECK_TIMEOUT=5
 OPENROUTER_API_KEY=
 OPENROUTER_MODEL=deepseek/deepseek-chat
 
+# Mistral OCR — translation of SCANNED PDFs (image-only pages).
+# Optional. If absent: scanned PDFs are rejected with an explicit error.
+# Get API key from: https://console.mistral.ai/  (~$1 / 1000 pages)
+MISTRAL_API_KEY=
+MISTRAL_OCR_MODEL=mistral-ocr-latest
+MISTRAL_OCR_TIMEOUT=180
+MISTRAL_OCR_ENABLED=true
+# Average extractable chars/page below which a PDF is treated as scanned
+SCANNED_PDF_MIN_CHARS_PER_PAGE=100
+
 # Ollama Configuration. Optional. If absent: default http://localhost:11434 (provider may be disabled).
 OLLAMA_BASE_URL=http://localhost:11434
 OLLAMA_MODEL=llama3