fix(make): auto-detect docker/podman so Makefile works on either engine

- Makefile: introduce CONTAINER_CMD ?= $(shell command -v docker || echo podman),
  replace all 12 hardcoded `docker` invocations with $(CONTAINER_CMD). User can
  override with `make <target> CONTAINER_CMD=podman` or env export.
- e2e/tests/us1-bootstrap-admin.spec.ts: AC-1.4 regex updated to match the new
  variable form `$(CONTAINER_CMD) exec … flask create-admin` (was hardcoded
  `docker exec`). RUNTIME default also auto-detects (same logic as Makefile)
  so the test exec'es the right engine without a MIMIC_CONTAINER_CMD export.
- e2e/tests/us6-deployment.spec.ts: same RUNTIME auto-detect so the make-dry-run
  regex assertions on lines 75 + 77 match what the Makefile actually emits on
  a podman-only host.
- README + CHANGELOG document the new behavior.

Fixes the user-reported issue: "Le makefile ne fonctionne pas sur ma machine
qui n'a que podman."

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -98,7 +98,7 @@ mimic/
 
 | Target | What it does |
 |---|---|
-| `make build` | Build the `mimic:latest` Docker image (multistage: Node → Python) |
+| `make build` | Build the `mimic:latest` container image (multistage: Node → Python). Uses `docker` if installed, otherwise `podman` — override with `make build CONTAINER_CMD=podman` |
 | `make start` | Start the container (port from `PORT`, default 5000; mounts `mimic-data` volume) |
 | `make stop` | Stop and remove the container |
 | `make restart` | `make stop && make start` — preserves the SQLite volume |