test(tier1): backend Docker image + env contract

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

testing/.env.tier1

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+GITEA_URL=http://gitea:3000
+GITEA_BOT_USER=rfc-bot
+GITEA_BOT_TOKEN=tier1-bot-token-PLACEHOLDER
+GITEA_ORG=wiggleverse
+META_REPO=ohm-content
+REGISTRY_REPO=
+OAUTH_CLIENT_ID=tier1-oauth-client-PLACEHOLDER
+OAUTH_CLIENT_SECRET=tier1-oauth-secret-PLACEHOLDER
+APP_URL=http://localhost:8080
+SECRET_KEY=tier1-not-secret
+DATABASE_PATH=/data/rfc-app.db
+OWNER_GITEA_LOGIN=owner
+GITEA_WEBHOOK_SECRET=tier1-webhook-secret
+ENABLED_MODELS=claude
+SMTP_HOST=mailpit
+SMTP_PORT=1025
+SMTP_STARTTLS=false
+EMAIL_FROM=rfc@example.test
+EMAIL_FROM_NAME=RFC Tier1
+EMAIL_ENABLED=true
+TURNSTILE_REQUIRED=false