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-backend/src/model/mod.rs

@@ -0,0 +1,28 @@
+//! Re-exports from `zesdex-entities` crate under the original module paths,
+//! plus local sub-modules (agent_def, msglog) that weren't extracted.
+
+// Module re-exports matching original `crate::model::*` paths
+pub mod session {
+    pub use zesdex_entities::seaorm::auth::session::*;
+}
+pub mod session_lock {
+    pub use zesdex_entities::seaorm::auth::session_lock::*;
+}
+pub mod settings {
+    pub use zesdex_entities::seaorm::common::settings::*;
+}
+pub mod app_config {
+    pub use zesdex_entities::seaorm::common::app_config::*;
+}
+pub mod store {
+    pub use zesdex_entities::seaorm::common::store::*;
+}
+pub mod editlog {
+    pub use zesdex_entities::seaorm::common::edit_log::*;
+}
+pub mod memory {
+    pub use zesdex_entities::seaorm::common::memory::*;
+}
+/// Local modules not extracted to workspace crates
+pub mod msglog;
+pub mod agent_def;