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/dto/mod.rs

@@ -0,0 +1,25 @@
+//! Re-exports from `zesdex-entities` (canonical types) and `zesdex-dto`
+//! (provider request/response) under the original module paths.
+//!
+//! Chat types come from the entities crate to avoid type duplication
+//! with `crate::model::conversation::Conversation` which stores
+//! `ChatMessage` values. Provider wire types come from the dto crate.
+
+pub mod chat {
+    pub mod message {
+        pub use zesdex_entities::seaorm::common::message::*;
+    }
+    pub mod tool {
+        pub use zesdex_entities::seaorm::common::tool_call::*;
+    }
+}
+
+pub mod provider {
+    pub mod request {
+        pub use zesdex_dto::provider::request::*;
+        pub use zesdex_dto::provider::request::ChatCompletionRequest as ChatRequest;
+    }
+    pub mod response {
+        pub use zesdex_dto::provider::response::ChatCompletionResponse as ChatResponse;
+    }
+}