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/Cargo.toml

@@ -0,0 +1,67 @@
+[package]
+name = "zesdex-backend"
+version.workspace = true
+edition.workspace = true
+authors.workspace = true
+
+[dependencies]
+# Workspace crates
+zesdex-entities = { path = "../zesdex-entities" }
+zesdex-utils = { path = "../zesdex-utils" }
+zesdex-dto = { path = "../zesdex-dto" }
+zesdex-ipc = { path = "../zesdex-ipc" }
+zesdex-iam = { path = "../zesdex-iam" }
+zesdex-cms = { path = "../zesdex-cms" }
+zesdex-middleware = { path = "../zesdex-middleware" }
+zesdex-libs = { path = "../zesdex-libs" }
+
+# External deps
+serde.workspace = true
+serde_json.workspace = true
+serde_yaml_ng.workspace = true
+chrono.workspace = true
+uuid.workspace = true
+anyhow.workspace = true
+tokio.workspace = true
+tracing.workspace = true
+tracing-subscriber.workspace = true
+reqwest.workspace = true
+ratatui.workspace = true
+crossterm.workspace = true
+rusqlite.workspace = true
+base64.workspace = true
+sha2.workspace = true
+hex.workspace = true
+libc.workspace = true
+dirs.workspace = true
+regex.workspace = true
+globset.workspace = true
+ignore.workspace = true
+nucleo-matcher.workspace = true
+futures-util.workspace = true
+rmcp.workspace = true
+lsp-types.workspace = true
+tiktoken-rs.workspace = true
+similar.workspace = true
+syntect.workspace = true
+pulldown-cmark.workspace = true
+infer.workspace = true
+webbrowser.workspace = true
+url.workspace = true
+percent-encoding.workspace = true
+dom_smoothie.workspace = true
+fast_html2md.workspace = true
+scraper.workspace = true
+include_dir.workspace = true
+
+[[bin]]
+name = "zesdex"
+path = "src/main.rs"
+
+[[bin]]
+name = "seed"
+path = "src/bin/seed.rs"
+
+[[bin]]
+name = "migrate"
+path = "src/bin/migrate.rs"