feat: simplify layout model selection and archive proposals

Changes:
- Replace PP-Structure 7-slider parameter UI with simple 3-option layout model selector
- Add layout model mapping: chinese (PP-DocLayout-S), default (PubLayNet), cdla
- Add LayoutModelSelector component and zh-TW translations
- Fix "default" model behavior with sentinel value for PubLayNet
- Add gap filling service for OCR track coverage improvement
- Add PP-Structure debug utilities
- Archive completed/incomplete proposals:
  - add-ocr-track-gap-filling (complete)
  - fix-ocr-track-table-rendering (incomplete)
  - simplify-ppstructure-model-selection (22/25 tasks)
- Add new layout model tests, archive old PP-Structure param tests
- Update OpenSpec ocr-processing spec with layout model requirements

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/core/config.py

@@ -3,7 +3,7 @@ Tool_OCR - Configuration Management
 Loads environment variables and provides centralized configuration
 """
 
-from typing import List
+from typing import List, Optional
 from pydantic_settings import BaseSettings
 from pydantic import Field
 from pathlib import Path
@@ -99,6 +99,33 @@ class Settings(BaseSettings):
     text_det_box_thresh: float = Field(default=0.3)  # Lower box threshold for better detection
     text_det_unclip_ratio: float = Field(default=1.2)  # Smaller unclip for tighter text boxes
 
+    # Layout Detection Model Configuration
+    # Available models:
+    # - None (default): Use PP-StructureV3's built-in model (PubLayNet-based)
+    # - "PP-DocLayout-S": Better for Chinese docs, papers, contracts, exams (23 categories)
+    # - "picodet_lcnet_x1_0_fgd_layout_cdla": CDLA-based model for Chinese document layout
+    layout_detection_model_name: Optional[str] = Field(
+        default="PP-DocLayout-S",
+        description="Layout detection model name. Set to 'PP-DocLayout-S' for better Chinese document support."
+    )
+    layout_detection_model_dir: Optional[str] = Field(
+        default=None,
+        description="Custom layout detection model directory. If None, downloads official model."
+    )
+
+    # ===== Gap Filling Configuration =====
+    # Supplements PP-StructureV3 output with raw OCR regions when detection is incomplete
+    gap_filling_enabled: bool = Field(default=True)  # Enable gap filling for OCR track
+    gap_filling_coverage_threshold: float = Field(default=0.7)  # Activate when coverage < 70%
+    gap_filling_iou_threshold: float = Field(default=0.15)  # IoU threshold for coverage detection
+    gap_filling_confidence_threshold: float = Field(default=0.3)  # Min confidence for raw OCR regions
+    gap_filling_dedup_iou_threshold: float = Field(default=0.5)  # IoU threshold for deduplication
+
+    # ===== Debug Configuration =====
+    # Enable debug outputs for PP-StructureV3 analysis
+    pp_structure_debug_enabled: bool = Field(default=True)  # Save debug files for PP-StructureV3
+    pp_structure_debug_visualization: bool = Field(default=True)  # Generate visualization images
+
     # Performance tuning
     use_fp16_inference: bool = Field(default=False)  # Half-precision (if supported)
     enable_cudnn_benchmark: bool = Field(default=True)  # Optimize convolution algorithms