docs: tambah doc comment, logging, dan inline comments di semua 255 file

Meliputi:
- File-level //! doc comment: tujuan file, alur kerja, komponen utama
- Function-level /// doc comment: apa, parameter, return, flow, edge cases
- Struct/enum/trait /// doc comment: peran, field docs
- Tracing logging (tracing::info!/debug!/trace!/warn!/error!) di setiap fungsi
- Inline comments untuk variable dan branching logic penting
- Seluruh 8 crates di workspace: zesdex-backend, zesdex-cms, zesdex-entities,
  zesdex-iam, zesdex-infra, zesdex-ipc, zesdex-middleware, zesdex-utils
- Build: 0 errors, 242/242 tests passed

crates/zesdex-backend/src/view/overlays/usage.rs

@@ -1,15 +1,26 @@
+//! Overlay: usage statistics — detailed token usage, API call count, edit/
+//! review/lesson activity counters, and session elapsed time.
+//!
+//! Flow: reads `state.session_runtime` and passes raw counters through
+//! `sidebar::compute_usage_summary` to get display-ready numbers, then
+//! renders them in an info-coloured overlay with two labelled sections:
+//! "Token Usage" and "Activity".
+
 use ratatui::style::{Modifier, Style};
 use ratatui::text::{Line, Span};
 use ratatui::widgets::{Block, Paragraph};
 use ratatui::Frame;
 use crate::view::theme::Theme;
+use tracing;
 
+/// Render the Usage overlay — token stats, activity counters, session time.
 pub fn render(
     frame: &mut Frame,
     area: ratatui::layout::Rect,
     block: Block<'static>,
     state: &crate::app::state::rest::AppStateRest,
 ) {
+    tracing::debug!("overlay: Usage");
     let block = block
         .title(Span::styled(
             " Usage ",