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.

Dockerfile

@@ -0,0 +1,30 @@
+# syntax=docker/dockerfile:1
+# Zesdex — Multi-stage Docker build
+# ===================================
+# Stage 1: Build with Rust toolchain
+FROM rust:1.85-slim-bookworm AS builder
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    pkg-config libsqlite3-dev && \
+    rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+COPY . .
+
+# Build with release profile (treats warnings as errors via lints)
+RUN cargo build --release -p zesdex-backend --bin zesdex
+
+# Stage 2: Minimal runtime image
+FROM debian:bookworm-slim
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    ca-certificates libsqlite3-0 && \
+    rm -rf /var/lib/apt/lists/*
+
+COPY --from=builder /app/target/release/zesdex /usr/local/bin/zesdex
+
+ENV ZESDEX_DATA_DIR=/data
+
+VOLUME ["/data"]
+
+ENTRYPOINT ["/usr/local/bin/zesdex"]