EPIC: Developer Experience, Cargo Xtask Profiles & CI/CD Pipeline Modernization

## Overview

This sub-EPIC focuses on eliminating developer friction, speeding up CI feedback loops, and providing matrix-level compile verification across all backend feature permutations.

## Goals

1. **Cargo Xtask Feature Matrix**: Adopt `cargo xtask` with support for a 9-profile matrix (`minimal`, `postgres`, `mysql`, `sqlite`, `aws`, `vault`, `gcp`, `azure`, `all`) and `check-profiles` matrix verification.
2. **Modular Local Development**: Introduce Docker Compose profiles (`db`, `observability`, `acme`, `aws`) so services can be spun up modularly.
3. **CI Decoupling**: Extract Helm and Kubernetes templating validation out of `CI.yml` into a dedicated `helm-checks.yml` workflow.
4. **Supply Chain Hardening**: Pin Dockerfile builder images by immutable `@sha256:...` digests.
5. **Local CI Mirroring**: Ensure `local-ci.sh` reliably mirrors remote GitHub Actions.