feat(ocr): fine-tune OCR quality criteria with stricter threshold and head-to-head comparison

- Raise quality acceptance threshold from 65→85 (configurable via TEXT_QUALITY_THRESHOLD)
- Reject text with significant issues (excessive_typos, garbage_characters,
  incoherent_text, fragmented_sentences) even when score is above threshold
  (configurable via TEXT_QUALITY_SIGNIFICANT_ISSUES)
- Add compare_text_quality() for AI-powered head-to-head comparison of
  original embedded text vs fresh OCR output
- Update process_document to pass original text to OCR task for comparison
- Update process_with_ocr to run comparison and keep the higher-quality text
- Add new settings to settings_service.py metadata
- Update docs/ConfigurationGuide.md with new settings
- Add comprehensive tests for new threshold and comparison logic

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

app/utils/settings_service.py

@@ -1070,6 +1070,31 @@ SETTING_METADATA = {
         "required": False,
         "restart_required": False,
     },
+    "text_quality_threshold": {
+        "category": "Processing",
+        "description": (
+            "Minimum quality score (0–100) required to accept embedded PDF text without re-OCR. "
+            "Text scoring below this threshold triggers a fresh OCR pass. "
+            "Default: 85. Lower values are more permissive; higher values enforce stricter quality."
+        ),
+        "type": "integer",
+        "sensitive": False,
+        "required": False,
+        "restart_required": False,
+    },
+    "text_quality_significant_issues": {
+        "category": "Processing",
+        "description": (
+            "Comma-separated list of quality issue labels that force OCR re-run even when the quality "
+            "score meets TEXT_QUALITY_THRESHOLD. Any matching issue in the AI assessment will trigger "
+            "re-OCR regardless of the numeric score. "
+            "Default: excessive_typos,garbage_characters,incoherent_text,fragmented_sentences"
+        ),
+        "type": "list",
+        "sensitive": False,
+        "required": False,
+        "restart_required": False,
+    },
     # Notifications Settings
     "notification_urls": {
         "category": "Notifications",