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.

shell.nix

@@ -0,0 +1,17 @@
+{ pkgs ? import <nixpkgs> { } }:
+
+with pkgs;
+
+mkShell {
+  buildInputs = [
+    (rust-bin.stable.latest.default.override {
+      extensions = [ "rust-src" "rust-analyzer" "clippy" ];
+    })
+    pkg-config
+    openssl
+    sqlite
+  ];
+
+  RUST_SRC_PATH = "${rust-bin.stable.latest.default}/lib/rustlib/src/rust/library";
+  RUST_LOG = "info";
+}