Align files service with architecture documentation

[rowan-stein]

User Request

Align the files service with the architecture documentation in agynio/architecture (commit 4fbedf1).

Specification

1. Dockerfile

• Add --platform=$BUILDPLATFORM on build stage
• Switch to Alpine base images with non-root user
• Add buf stage for proto generation
• Standard ARG/ENV pattern for cross-compilation

2. Workflows

Merge docker-ghcr.yml + helm-release.yml into single release.yml:

• Tag-only trigger (v*.*.*) — remove main push trigger
• Keep metadata-action tags (remove main tag rule)
• Keep GHA layer cache
• Add manifest verification step with jq
• Combine image + helm jobs

3. buf.gen.yaml / Proto

• Output dir: gen/go → .gen/go
• Update .gitignore, all Go import paths

4. Helm Chart

• Already has service-base ✅
• Remove duplicate templates (evaluate which are passthroughs)

5. DevSpace (missing)

Create devspace.yaml with e2e-runner.

6. E2E Tests (missing)

Create test/e2e/ with //go:build e2e tags.

7. README (missing)

Create standard template.

Reference

Follow aligned repos: gateway, chat, notifications, authorization, teams, llm.