feat: restructure into Cargo workspace for microservices

[raphaeldiscky]

Summary

• Restructure flat single-crate template into a Cargo workspace monorepo with Clean Architecture layers
• crates/app-core: shared infrastructure (config via figment, error handling, PostgreSQL pool, JWT auth, input validation, observability)
• crates/app-user: user domain with entity → dto → repository trait → service → handler → routes
• crates/app-testing: testcontainers-rs helpers for integration testing against real PostgreSQL
• services/user-service: thin binary entrypoint with AppState + FromRef DI
• deployments/docker-compose: PostgreSQL with external network, infra composition
• scripts/migrate.sh: per-service migration script reading DATABASE_URL from each service's .env
• Integration tests using testcontainers (6 tests, all passing)

Architecture

crates/app-core/        → shared infrastructure (config, error, db, auth, validation)
crates/app-user/        → user domain (Clean Architecture layers)
crates/app-testing/     → testcontainers helpers
services/user-service/  → binary entrypoint
deployments/            → docker-compose files

Test plan

• cargo clippy --workspace -- -D warnings passes clean
• cargo nextest run --workspace — 6 integration tests pass (requires Docker)
• task start_infra starts PostgreSQL on port 15432
• task db:migrate runs migrations via scripts/migrate.sh
• cargo run -p user-service starts server (requires .env with DATABASE_URL, JWT_SECRET, PORT)
• Verify CRUD endpoints: POST/GET/PUT/DELETE /api/users
• Verify health endpoints: GET /healthz, GET /readyz