feat(ocr): add multi-language OCR support with per-pipeline language override

- Add OCR_LANGUAGES constant (28 languages, EN/DE/FR/ES/IT/PT/RU/ZH/JA/KO/AR/etc.)
- Add TESSERACT_TO_EASYOCR mapping for automatic code translation
- Add optional language constructor arg to TesseractOCRProvider/EasyOCRProvider
- Update get_ocr_providers() to accept and pass per-call language override
- Add language parameter to process_with_ocr Celery task
- Add _get_pipeline_ocr_language() helper to resolve OCR language from pipeline step config
- Update process_document to look up and pass pipeline OCR language to process_with_ocr
- Add ocr_language select config field (28 options) to pipeline OCR step schema
- Add language dropdown to pipeline UI (pipelines.html)
- Update docs/UserGuide.md and docs/API.md with language override documentation
- Add 27 new tests covering language constants, provider overrides, and pipeline lookup

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

docs/API.md

@@ -1785,12 +1785,27 @@ Returns the catalogue of built-in step types.
     "label": "OCR Processing",
     "description": "Extract text using Azure Document Intelligence or local Tesseract.",
     "config_schema": {
-      "force_cloud_ocr": { "type": "boolean", "default": false }
+      "force_cloud_ocr": { "type": "boolean", "default": false },
+      "ocr_language": {
+        "type": "select",
+        "default": "auto",
+        "description": "Language(s) for OCR. Overrides the global setting for Tesseract/EasyOCR. Azure/Mistral auto-detect.",
+        "options": [
+          { "value": "auto", "label": "Auto (use system default)" },
+          { "value": "eng",  "label": "English" },
+          { "value": "deu",  "label": "German" },
+          { "value": "fra",  "label": "French" },
+          { "value": "spa",  "label": "Spanish" },
+          "..."
+        ]
+      }
     }
   }
 }
 ```
 
+The `ocr_language` field accepts Tesseract language codes (e.g. `"eng"`, `"deu"`, `"eng+deu"` for multi-language) or `"auto"` to fall back to the global system setting.  The full list of 28 supported language codes is returned by the step-types endpoint.
+
 ### List pipelines
 
 ```bash
@@ -1883,12 +1898,23 @@ Content-Type: application/json
 
 {
   "step_type": "ocr",
-  "label": "Cloud OCR",
-  "config": { "force_cloud_ocr": true },
+  "label": "German OCR",
+  "config": { "force_cloud_ocr": false, "ocr_language": "deu" },
   "enabled": true
 }
 ```
 
+Multi-language (Tesseract `+`-separated codes):
+
+```bash
+{
+  "step_type": "ocr",
+  "config": { "ocr_language": "eng+deu" }
+}
+```
+
+Use `"ocr_language": "auto"` (or omit the field) to fall back to the global system language setting.
+
 ### Update step
 
 ```bash