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_language_validation.py

@@ -0,0 +1,45 @@
+"""LanguageValidator — case-insensitive codes and canonical form (zh-CN fix)."""
+
+import pytest
+
+from middleware.validation import LanguageValidator, ValidationError
+
+
+class TestLanguageValidatorCaseInsensitive:
+    def test_zh_cn_mixed_case_accepted(self):
+        assert LanguageValidator.validate("zh-CN") == "zh-CN"
+
+    def test_zh_cn_lowercase_normalized(self):
+        assert LanguageValidator.validate("zh-cn") == "zh-CN"
+
+    def test_zh_cn_uppercase_normalized(self):
+        assert LanguageValidator.validate("ZH-CN") == "zh-CN"
+
+    def test_zh_tw_accepted(self):
+        assert LanguageValidator.validate("zh-TW") == "zh-TW"
+        assert LanguageValidator.validate("zh-tw") == "zh-TW"
+
+    def test_alias_chinese(self):
+        assert LanguageValidator.validate("chinese") == "zh-CN"
+
+    def test_alias_tw(self):
+        assert LanguageValidator.validate("tw") == "zh-TW"
+
+    def test_plain_codes_unchanged(self):
+        assert LanguageValidator.validate("en") == "en"
+        assert LanguageValidator.validate("fr") == "fr"
+
+    def test_auto_accepted(self):
+        assert LanguageValidator.validate("auto") == "auto"
+
+    def test_unknown_code_rejected(self):
+        with pytest.raises(ValidationError):
+            LanguageValidator.validate("xx")
+
+    def test_unknown_variant_rejected(self):
+        with pytest.raises(ValidationError):
+            LanguageValidator.validate("zz-ZZ")
+
+    def test_empty_rejected(self):
+        with pytest.raises(ValidationError):
+            LanguageValidator.validate("")