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/service.rs

@@ -0,0 +1,53 @@
+//! Service trait definitions — use-case boundaries for CMS operations.
+//!
+//! These traits are implemented by the application layer and consumed by
+//! infrastructure adapters (HTTP handlers, CLI commands, etc.).
+
+#![allow(
+    clippy::cast_possible_truncation,
+    clippy::cast_sign_loss,
+    clippy::cast_precision_loss,
+    clippy::cast_possible_wrap
+)]
+
+use anyhow::Result;
+
+use super::conversation::{ChatMessage, Conversation};
+use super::memory::Memory;
+use super::settings::Settings;
+
+/// Settings use cases.
+pub trait SettingsService {
+    /// Load current settings from the default store.
+    fn load_settings(&self) -> Result<Settings>;
+
+    /// Persist updated settings.
+    fn save_settings(&self, settings: &Settings) -> Result<()>;
+
+    /// Update the provider configuration (name and details).
+    fn update_provider(&self, name: &str, config: &super::app_config::ProviderConfig) -> Result<()>;
+}
+
+/// Conversation management use cases.
+pub trait ConversationService {
+    /// Load a conversation for the given session id.
+    fn load_conversation(&self, session_id: &str) -> Result<Conversation>;
+
+    /// Persist a conversation.
+    fn save_conversation(&self, conv: &Conversation) -> Result<()>;
+
+    /// Append a single message and persist.
+    fn add_message(&self, conv: &mut Conversation, msg: ChatMessage) -> Result<()>;
+}
+
+/// Memory management use cases.
+pub trait MemoryService {
+    /// List all memory slugs.
+    fn list_memories(&self) -> Result<Vec<String>>;
+
+    /// Save (create or update) a memory.
+    fn save_memory(&self, memory: &Memory) -> Result<()>;
+
+    /// Delete a memory by name.
+    fn delete_memory(&self, name: &str) -> Result<()>;
+}