feat: enable document orientation detection for scanned PDFs

- Enable PP-StructureV3's use_doc_orientation_classify feature
- Detect rotation angle from doc_preprocessor_res.angle
- Swap page dimensions (width <-> height) for 90°/270° rotations
- Output PDF now correctly displays landscape-scanned content

Also includes:
- Archive completed openspec proposals
- Add simplify-frontend-ocr-config proposal (pending)
- Code cleanup and frontend simplification

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

openspec/changes/archive/2025-12-11-cleanup-dead-code/specs/document-processing/spec.md

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+## REMOVED Requirements
+
+### Requirement: Legacy PDF Generator Service
+
+**Reason**: `pdf_generator.py` (507 lines) was the original PDF generation implementation using Pandoc/WeasyPrint. It has been completely superseded by `pdf_generator_service.py` which uses ReportLab for low-level PDF generation with full layout preservation, table rendering, and image support.
+
+**Migration**: No migration needed. The new `pdf_generator_service.py` provides all functionality with improved features.
+
+#### Scenario: Legacy PDF generator file removal
+- **WHEN** the legacy `pdf_generator.py` file is removed
+- **THEN** the system continues to function normally using `pdf_generator_service.py`
+- **AND** PDF generation works correctly with layout preservation
+- **AND** no import errors occur in any service or router
+
+### Requirement: Deprecated IoU Configuration Parameters
+
+**Reason**: `gap_filling_iou_threshold` and `gap_filling_dedup_iou_threshold` are deprecated configuration parameters that should be replaced by IoA (Intersection over Area) thresholds for better accuracy.
+
+**Migration**: Use `gap_filling_dedup_ioa_threshold` instead.
+
+#### Scenario: Deprecated config removal
+- **WHEN** the deprecated IoU configuration parameters are removed from config.py
+- **THEN** gap filling service uses IoA-based thresholds
+- **AND** the system starts without configuration errors
+
+## ADDED Requirements
+
+### Requirement: Unified Bbox Utility Module
+
+The system SHALL provide a centralized bbox utility module (`backend/app/utils/bbox_utils.py`) for consistent bounding box normalization across all services.
+
+#### Scenario: Bbox normalization from polygon format
+- **WHEN** a bbox in polygon format `[[x1,y1], [x2,y2], [x3,y3], [x4,y4]]` is provided
+- **THEN** the utility returns normalized tuple `(x0, y0, x1, y1)` representing min/max coordinates
+
+#### Scenario: Bbox normalization from flat array
+- **WHEN** a bbox in flat array format `[x0, y0, x1, y1]` is provided
+- **THEN** the utility returns normalized tuple `(x0, y0, x1, y1)`
+
+#### Scenario: Bbox normalization from 8-point polygon
+- **WHEN** a bbox in 8-point format `[x1, y1, x2, y2, x3, y3, x4, y4]` is provided
+- **THEN** the utility calculates and returns normalized tuple `(min_x, min_y, max_x, max_y)`