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.

.github/workflows/ci.yml

@@ -21,11 +21,11 @@ jobs:
         with:
           components: clippy
 
-      - name: Build
-        run: cargo build --release
+      - name: Build workspace
+        run: cargo build --release --workspace
 
-      - name: Test
-        run: cargo test
+      - name: Test workspace
+        run: cargo test --workspace
 
-      - name: Clippy
-        run: cargo clippy -- -D warnings
+      - name: Clippy workspace
+        run: cargo clippy --workspace -- -D warnings