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/db/mod.rs

@@ -0,0 +1,57 @@
+pub mod projects;
+pub mod settings;
+pub mod tags;
+
+// Re-export all types and functions
+pub use projects::*;
+pub use settings::*;
+pub use tags::*;
+
+use sqlx::{PgPool, postgres::PgPoolOptions};
+
+/// Database connection pool creation
+pub async fn create_pool(database_url: &str) -> Result<PgPool, sqlx::Error> {
+    PgPoolOptions::new()
+        .max_connections(20)
+        .acquire_timeout(std::time::Duration::from_secs(3))
+        .connect(database_url)
+        .await
+}
+
+/// Health check query
+pub async fn health_check(pool: &PgPool) -> Result<(), sqlx::Error> {
+    sqlx::query!("SELECT 1 as check")
+        .fetch_one(pool)
+        .await
+        .map(|_| ())
+}
+
+/// Slugify text for URL-safe identifiers
+pub fn slugify(text: &str) -> String {
+    text.to_lowercase()
+        .chars()
+        .map(|c| {
+            if c.is_alphanumeric() {
+                c
+            } else if c.is_whitespace() || c == '-' || c == '_' {
+                '-'
+            } else {
+                '\0'
+            }
+        })
+        .collect::<String>()
+        .split('-')
+        .filter(|s| !s.is_empty())
+        .collect::<Vec<_>>()
+        .join("-")
+}
+
+/// Project status enum
+#[derive(Debug, Clone, Copy, PartialEq, Eq, sqlx::Type, serde::Serialize, serde::Deserialize)]
+#[sqlx(type_name = "project_status", rename_all = "lowercase")]
+pub enum ProjectStatus {
+    Active,
+    Maintained,
+    Archived,
+    Hidden,
+}