refactor(llm-api): implement clean architecture following scraper pattern

Split monolithic 1012-line main.rs into layered hexagonal architecture:
- Domain: entity types and LlmError enum
- Application: prompt building, sampler construction, tool call parsing
- Infrastructure: LlamaEngine wrapping llama-cpp-2 with isolated unsafe transmute
- Presentation: Axum handlers, middleware (auth), error chain, router
- Config: type-safe AppConfig with LazyLock
- Bootstrap: Application struct with build() + run()

Resolves build_sampler/build_sampler_params duplication.
Adds simple web chat UI at GET /.

Co-Authored-By: Claude Code <noreply@anthropic.com>

src/config/mod.rs

@@ -0,0 +1,59 @@
+//! Type-safe application configuration.
+//!
+//! Loads configuration from environment variables at startup with fail-fast behavior.
+
+use std::sync::LazyLock;
+
+const DEFAULT_MODEL_PATH: &str = "/models/MiniCPM-V-4.6-Q4_K_M.gguf";
+pub const MODEL_ID: &str = "minicpm-v-4.6";
+
+/// Application configuration loaded at startup from environment variables.
+#[derive(Debug, Clone)]
+pub struct AppConfig {
+    /// Path to the GGUF model file
+    pub model_path: String,
+
+    /// API key for authentication (empty = disabled)
+    pub api_key: String,
+
+    /// Server port to bind to
+    pub server_port: u16,
+
+    /// Log level (trace, debug, info, warn, error)
+    pub log_level: String,
+
+    /// LLM context size (n_ctx)
+    pub n_ctx: u32,
+
+    /// LLM batch size (n_batch)
+    pub n_batch: u32,
+
+    /// Number of CPU threads for inference
+    pub n_threads: i32,
+}
+
+impl AppConfig {
+    /// Load configuration from environment variables.
+    pub fn load() -> Self {
+        Self {
+            model_path: std::env::var("MODEL_PATH")
+                .unwrap_or_else(|_| DEFAULT_MODEL_PATH.to_string()),
+            api_key: std::env::var("API_KEY").unwrap_or_default(),
+            server_port: std::env::var("SERVER_PORT")
+                .ok()
+                .and_then(|v| v.parse().ok())
+                .unwrap_or(8080),
+            log_level: std::env::var("RUST_LOG").unwrap_or_else(|_| "info".to_string()),
+            n_ctx: 2048,
+            n_batch: 512,
+            n_threads: 4,
+        }
+    }
+}
+
+/// Global configuration instance, loaded once at startup.
+pub static CONFIG: LazyLock<AppConfig> = LazyLock::new(|| {
+    let config = AppConfig::load();
+    tracing::info!("Configuration loaded: model={:?}", config.model_path);
+    config
+});