feat(server): tenant-registry skeleton boots against dev stack

Minimal Go service so platform/portal has something to resolve in local
dev. Stdlib net/http with Go 1.22 enhanced ServeMux (method+path
patterns); no third-party deps yet.

Layout:
  cmd/server/main.go               entry point with graceful shutdown
  internal/config/                 env-driven config (APP_ENV, ADDR, KC issuer)
  internal/server/                 http handlers + request-logging middleware
  internal/store/memory.go         in-memory tenant store, seeded with acme
  migrations/0001_init.up.sql      schema for the M4.1 follow-up (unapplied)
  Makefile                         dev/test/build/lint/docker targets
  Dockerfile                       multi-stage distroless build

Endpoints (under :8080 in dev):
  GET /healthz
  GET /v1/tenants/by-slug/{slug}   200 acme | 404
  GET /v1/tenants/{id}             200 by uuid | 404

JWT validation and the real Postgres-backed store land in the M4.1
follow-up PR — keeping this PR strictly to 'boots, replies, tests pass'.

Refs: M4.1 (skeleton)

CHANGELOG.md

@@ -6,6 +6,7 @@ Generated section is appended on release tag via `git-cliff` (see `.gitea/workfl
 ## [Unreleased]
 
 ### Added
+- feat(server): minimal Go service — /healthz + GET /v1/tenants/by-slug/:slug + GET /v1/tenants/:id with in-memory store seeded with the acme tenant
 -
 
 ### Changed