refactor: reorganize Rust codebase into modular handlers and database layers

- Split monolithic src/db.rs (1122 lines) into domain modules: projects, tags, settings
- Extract API handlers from main.rs into separate handler modules by domain
- Add proxy module for ISR/SSR coordination with Bun process
- Introduce AppState for shared application context
- Add utility functions for asset serving and request classification
- Remove obsolete middleware/auth.rs in favor of session checks in handlers

src/state.rs

@@ -0,0 +1,33 @@
+use std::sync::Arc;
+
+use crate::{auth::SessionManager, health::HealthChecker, tarpit::TarpitState};
+
+/// Application state shared across all handlers
+#[derive(Clone)]
+pub struct AppState {
+    pub downstream_url: String,
+    pub http_client: reqwest::Client,
+    pub unix_client: Option<reqwest::Client>,
+    pub health_checker: Arc<HealthChecker>,
+    pub tarpit_state: Arc<TarpitState>,
+    pub pool: sqlx::PgPool,
+    pub session_manager: Arc<SessionManager>,
+}
+
+/// Errors that can occur during proxying to Bun
+#[derive(Debug)]
+pub enum ProxyError {
+    Network(reqwest::Error),
+    Other(String),
+}
+
+impl std::fmt::Display for ProxyError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            ProxyError::Network(e) => write!(f, "Network error: {e}"),
+            ProxyError::Other(s) => write!(f, "{s}"),
+        }
+    }
+}
+
+impl std::error::Error for ProxyError {}