Add text-to-image generation support

Implemented complete text-to-image functionality across all AI providers:

Backend additions:
- Added text_to_image() method to AIProvider abstract class
- Implemented for all providers:
  * OpenAI: DALL-E generations API
  * Stability AI: SDXL text-to-image with negative prompts
  * Replicate: SDXL with full parameter control
  * Mock: Placeholder image generation for testing

New API endpoints (/generate):
- POST /generate/text-to-image
  * Generate image from prompt
  * Optional: create new project automatically
  * Configurable width/height (256-2048px)
  * Negative prompt support
  * Provider and model selection

- POST /generate/layer/text-to-image
  * Generate image as layer in existing project
  * Smaller dimensions for layer composition
  * Position control (x, y coordinates)
  * Saves to project layers directory

Features:
- Full provider support (OpenAI, Stability, Replicate, Mock)
- Negative prompts for better control
- Auto-project creation option
- Layer-based generation for compositing
- Dimension validation (256-2048px range)
- Model selection per request

Use cases:
- Create new images from scratch
- Generate elements to add as layers
- Quick ideation and iteration
- Base image creation for further editing

Next: Advanced canvas UI with layers and real-time preview

backend/app/schemas.py

@@ -107,6 +107,30 @@ class PatchApply(BaseModel):
     feather_px: int = 5
 
 
+# Text-to-Image schemas
+class TextToImageRequest(BaseModel):
+    prompt: str
+    width: int = 1024
+    height: int = 1024
+    negative_prompt: Optional[str] = None
+    ai_provider: Optional[str] = None
+    ai_model: Optional[str] = None
+    create_project: bool = True
+    project_name: Optional[str] = None
+
+
+class TextToImageResponse(BaseModel):
+    status: str
+    prompt: str
+    width: int
+    height: int
+    project_id: Optional[int] = None
+    image_url: Optional[str] = None
+    layer_position: Optional[Dict[str, int]] = None
+    ai_provider: str
+    ai_model: Optional[str] = None
+
+
 # Generic responses
 class StatusResponse(BaseModel):
     status: str