feat: Improve image generation prompts with visual data extraction

- Add dedicated image_generation module with statistical extraction
- Support 16 industry domains with visual concept detection
- Add model-specific guidance for Ideogram, FLUX, GLM, Qwen, MAI
- Extract statistics, rankings, comparisons, and trends automatically
- Refactor backend/api/images.py to use new module

backend/services/image_generation/__init__.py

@@ -0,0 +1,22 @@
+"""
+Image Generation Services.
+
+This package provides services for AI-powered image generation,
+including visual data extraction and prompt optimization.
+"""
+
+from .visual_data_extractor import (
+    extract_visual_data,
+    get_model_recommendation,
+    build_visual_summary,
+    ExtractedVisualData,
+    DOMAIN_VISUAL_CONCEPTS,
+)
+
+__all__ = [
+    "extract_visual_data",
+    "get_model_recommendation", 
+    "build_visual_summary",
+    "ExtractedVisualData",
+    "DOMAIN_VISUAL_CONCEPTS",
+]