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-entities/src/domain/common/mod.rs

@@ -1,6 +1,20 @@
-//! Common entity types shared across the Zesdex application: application
-//! configuration, settings, store paths, conversations, messages, tool
-//! calls, usage stats, and SSE streaming types.
+//! Common entity types shared across the Zesdex application.
+//!
+//! Contains pure data structures for conversations, messages, tool calls,
+//! usage statistics, provider API types (chat request/response, SSE stream),
+//! and store path configuration. All types derive `Serialize`/`Deserialize`
+//! and are persisted as JSON files.
+//!
+//! # Sub-modules
+//!
+//! - [`conversation`] — Ordered conversation (vector of `ChatMessage`)
+//! - [`message`] — `ChatMessage` + `Role` enum
+//! - [`provider`] — LLM provider API types: `ChatRequest`, `ChatResponse`,
+//!   `StreamEvent`, `SseParser`, `ToolDef`, etc.
+//! - [`store`] — `Store` paths for data directories
+//! - [`tool_call`] — `ToolCall` + `ToolFunction` (function-calling request)
+//! - [`tool_result`] — `ToolCallResult` (function-calling response)
+//! - [`usage`] — `UsageStats` (token counts, costs)
 
 pub mod conversation;
 pub mod message;