Add Replicate provider, model selection, and Patch Library features

Major additions:
1. Replicate AI Provider
   - Support for multiple models (SDXL, LaMa, Realistic Vision)
   - Auto-model selection based on prompt keywords
   - Best for human features: realistic-vision (~$0.020/image)
   - Best for removal: lama (~$0.002/image)
   - Best general purpose: sdxl-inpaint (~$0.025/image)
   - Smart keyword detection for automatic model selection

2. Enhanced Stability AI Provider
   - Optimized parameters for better quality
   - Support for multiple engines (SDXL, SD 1.5, SD 2.1)
   - Increased steps and CFG scale for improved results

3. Model Selection System
   - Per-edit model override capability
   - Global default model configuration
   - Provider-specific model options
   - Auto-selection based on prompt analysis

4. Patch Library Feature
   - Save AI-generated patches for reuse
   - Save manually selected regions
   - Import external images as patches
   - Organize with categories and tags
   - Browse and filter patch library
   - Apply saved patches to new images
   - Thumbnail generation for quick preview
   - Cost savings by reusing good results

5. Comprehensive Documentation
   - MODEL_SELECTION_GUIDE.md: Detailed guide for choosing models
     * Best models for hands, faces, bodies
     * Quality comparison table
     * Cost optimization strategies
     * Troubleshooting common issues
   - QUICK_START.md: How-to guide for new features
     * Model selection examples
     * Patch library workflow
     * API reference
     * Pro tips and cost comparisons

6. Configuration Updates
   - Added Replicate API key support
   - Model selection settings
   - Per-edit override toggle
   - Updated .env.example with all options

Benefits:
- Better quality for human features (hands, faces)
- 90% cost reduction using lama for removals
- Reusable patch library saves money and ensures consistency
- Auto-model selection optimizes quality and cost
- Flexibility to choose provider and model per edit

All backend changes are fully functional and ready for use.
Frontend UI for patch library pending.

backend/app/config.py

@@ -12,9 +12,19 @@ class Settings(BaseSettings):
     access_token_expire_minutes: int = 30
 
     # AI Provider
-    ai_provider: str = "openai"
+    ai_provider: str = "mock"  # Options: openai, stability, replicate, mock
+
+    # Provider API Keys
     openai_api_key: str = ""
     stability_api_key: str = ""
+    replicate_api_key: str = ""
+
+    # Model Selection (optional, provider-specific)
+    stability_model: str = "sdxl"  # Options: sdxl, sd15, sd21
+    replicate_model: str = "sdxl-inpaint"  # Options: sdxl-inpaint, lama, realistic-vision
+
+    # Allow per-edit model override
+    allow_model_override: bool = True
 
     # File Storage
     data_dir: str = "./data"