refactor: migrate monolithic crate to Cargo Workspace with Clean Architecture

Transform the single binary crate into a 9-crate workspace monorepo:

- Root Cargo.toml as [workspace] manager with resolver = "2"
- zesdex-entities: Domain entity types (session, settings, store, message, etc.)
- zesdex-utils: Pure utility functions (error, logger, pagination, slug, clipboard)
- zesdex-dto: Data Transfer Objects for LLM provider API communication
- zesdex-ipc: Unix-socket IPC layer (client/server/framing/protocol)
- zesdex-iam: Identity & Access Management (Clean Architecture: domain/application/infrastructure)
- zesdex-cms: Content Management (Clean Architecture: domain/application/infrastructure)
- zesdex-middleware: HTTP middleware (Auth, CORS, Rate Limiting)
- zesdex-libs: Composition root (AppContext, DB init, JWT, Argon2)
- zesdex-backend: Main binary entry point + seed/migrate binaries
- DevOps: Dockerfile, docker-compose, Nix (flake/shell/default), CI/CD updates
- Remove dead root src/ and src-misc/ directories

All crate re-exports maintain backward compatibility with original
crate::model::*, crate::dto::*, crate::ipc::* module paths.
Feature crates enforce strict layer separation: domain -> application
-> infrastructure with generic trait-based dependency injection.

crates/zesdex-cms/src/domain/mod.rs

@@ -0,0 +1,38 @@
+//! Domain layer — pure entities, repository traits, and service trait definitions.
+//!
+//! This layer has zero infrastructure dependencies; all I/O is expressed through
+//! repository traits defined in [`repository`].
+
+#![allow(
+    clippy::cast_possible_truncation,
+    clippy::cast_sign_loss,
+    clippy::cast_precision_loss,
+    clippy::cast_possible_wrap
+)]
+
+pub mod app_config;
+pub mod conversation;
+pub mod edit_log;
+pub mod memory;
+pub mod repository;
+pub mod service;
+pub mod settings;
+
+pub use app_config::AppConfig;
+pub use app_config::ModelRole;
+pub use app_config::ProviderConfig;
+pub use conversation::Conversation;
+pub use edit_log::EditLog;
+pub use edit_log::EditLogEntry;
+pub use memory::Memory;
+pub use repository::AppConfigRepository;
+pub use repository::ConversationRepository;
+pub use repository::EditLogRepository;
+pub use repository::MemoryRepository;
+pub use repository::SettingsRepository;
+pub use service::ConversationService;
+pub use service::MemoryService;
+pub use service::SettingsService;
+pub use settings::InternetMode;
+pub use settings::Settings;
+pub use settings::SettingsFlags;