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-entities/src/seaorm/common/mod.rs

@@ -0,0 +1,29 @@
+//! Common entity types shared across the Zesdex application: application
+//! configuration, settings, store paths, conversations, messages, tool
+//! calls, usage stats, and SSE streaming types.
+
+pub mod app_config;
+pub mod conversation;
+pub mod edit_log;
+pub mod memory;
+pub mod message;
+pub mod provider;
+pub mod settings;
+pub mod store;
+pub mod tool_call;
+pub mod tool_result;
+pub mod usage;
+
+pub use app_config::{AppConfig, ModelRole, ProviderConfig};
+pub use conversation::Conversation;
+pub use edit_log::{EditLog, EditLogEntry};
+pub use memory::Memory;
+pub use message::{ChatMessage, Role};
+pub use provider::{
+    ChatRequest, ChatResponse, Choice, SseParser, StreamEvent, StreamOptions, ToolDef, ToolFunctionDef,
+};
+pub use settings::{InternetMode, Settings, SettingsFlags};
+pub use store::Store;
+pub use tool_call::{ToolCall, ToolFunction};
+pub use tool_result::ToolCallResult;
+pub use usage::UsageStats;