📚 Add comprehensive API documentation

[Gilfeather]

Description

Create detailed API documentation with examples and OpenAPI/Swagger specification.

Acceptance Criteria

• Add OpenAPI 3.0 specification file (openapi.yaml)
• Document all endpoints with request/response schemas
• Include example requests and responses
• Add error code documentation
• Generate HTML documentation from spec
• Add link to docs in README

Current API Endpoints to Document

1. GET /healthz - Health check
2. GET /model/info - Model information and statistics
3. POST /predict - Inference endpoint

Updated API Specifications

Since the API now uses built-in models (ResNet18), documentation should reflect:

Model Info Response:

• Built-in model: resnet18
• Input shape: [1, 3, 224, 224] (150,528 elements)
• Output shape: [1000] (ImageNet classes)
• Backend: burn-resnet18

Predict Request:

• Single:
• Batch:

Error Codes:

• 400: INPUT_VALIDATION_FAILED
• 500: INFERENCE_FAILED
• 503: MODEL_NOT_LOADED

Implementation Guidance

• Use utoipa crate for Rust-native OpenAPI generation
• Add documentation attributes to API handlers in src/api.rs
• Include realistic ResNet18 example data (150,528 input values)
• Consider using Swagger UI for interactive docs
• Update examples to use built-in models instead of file paths

Estimated Difficulty

Medium - 1-2 days (updated for current API structure)