test(tier1): docker compose stack (gitea seed + backend + web + mailpit)

Wires the four-service Tier-1 integration stack and brings it up green.

Fixes found during live bring-up:
- gitea-admin-init: run as user 'git'; the gitea CLI refuses to run as
  root and our custom entrypoint bypasses the image's s6 init that
  normally drops privileges.
- backend.Dockerfile: COPY VERSION to /VERSION; health.py reads the
  canonical VERSION file from the repo root (parents[2]) at import, so
  it must exist in the image or the backend crashes on startup.

env_file cold-start: generated/.env.tier1.generated is created as an
empty placeholder (gitignored) so the backend env_file path always
exists; the seeder overwrites it before backend starts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Makefile

@@ -0,0 +1,14 @@
+tier1-up:
+	docker compose -f testing/docker-compose.yml up --build -d
+
+tier1-down:
+	docker compose -f testing/docker-compose.yml down -v
+
+tier1-logs:
+	docker compose -f testing/docker-compose.yml logs -f
+
+fe-unit:
+	cd frontend && npm run test:run
+
+e2e:
+	cd e2e && BASE_URL=$${BASE_URL:-http://localhost:8080} MAILSINK_URL=$${MAILSINK_URL:-http://localhost:8025} npm run e2e