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/handlers/mod.rs

@@ -0,0 +1,35 @@
+pub mod assets;
+pub mod auth;
+pub mod health;
+pub mod projects;
+pub mod settings;
+pub mod tags;
+
+// Re-export handlers for easier imports
+pub use assets::*;
+pub use auth::*;
+pub use health::*;
+pub use projects::*;
+pub use settings::*;
+pub use tags::*;
+
+// Request/Response types used by handlers
+
+#[derive(serde::Deserialize)]
+pub struct CreateTagRequest {
+    pub name: String,
+    pub slug: Option<String>,
+    pub color: Option<String>,
+}
+
+#[derive(serde::Deserialize)]
+pub struct UpdateTagRequest {
+    pub name: String,
+    pub slug: Option<String>,
+    pub color: Option<String>,
+}
+
+#[derive(serde::Deserialize)]
+pub struct AddProjectTagRequest {
+    pub tag_id: String,
+}