🔧 Add configuration file support

Description

Add support for loading server configuration from a TOML file instead of only command-line arguments.

Acceptance Criteria

• Create Config struct in new src/config.rs file
• Support loading from furnace.toml file
• Include settings: port, host, model_name, log_level, backend, max_concurrent_requests
• Command-line args should override config file values
• Add example furnace.toml file to repository
• Update CLI help text to mention config file

Updated Configuration Items

Since the CLI has been updated to use built-in models, the config should include:

• model_name (instead of model_path)
• backend (cpu, wgpu, metal, cuda)
• enable_kernel_fusion
• enable_autotuning
• max_concurrent_requests

Example furnace.toml

[server]
host = "127.0.0.1"
port = 3000
log_level = "info"
max_concurrent_requests = 100

[model]
name = "resnet18"
backend = "cpu"
enable_kernel_fusion = false
enable_autotuning = false

Implementation Guidance

• Use the toml crate for parsing
• Use serde for deserialization
• Modify main.rs to check for config file before parsing CLI args
• Use clap's value precedence features
• Update for current CLI structure with --model-name

Estimated Difficulty

Medium - 1-2 days