fix: migration atomicity + deploy hardening + Litestream R2

Migration atomicity:
- Remove conn.commit() and executescript() from all up() functions (0000,
  0011, 0012, 0013, 0014, 0015); executescript() issued implicit COMMITs
  which broke the batch-rollback guarantee of the migration runner
- Rewrite 0000 with individual conn.execute() calls (was a single
  executescript block)

Deploy hardening:
- Add pre-migration DB backup step to deploy.sh: saves
  app.db.pre-deploy-<timestamp> in the volume before every migration
- On health-check failure: restore the backup, then stop + exit
- On success: clean up old backups (keep last 3)

Litestream:
- Enable R2 as primary replica in litestream.yml (env-var placeholders)
- Add local /app/data/backups as secondary replica
- docker-compose: add auto-restore on empty volume (sh entrypoint runs
  'litestream restore' before 'litestream replicate' if app.db missing)
- Add LITESTREAM_R2_* vars to .gitlab-ci.yml .env block and .env.example

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitlab-ci.yml

@@ -48,6 +48,10 @@ deploy:
       PADDLE_WEBHOOK_SECRET=$PADDLE_WEBHOOK_SECRET
       PADDLE_PRICE_STARTER=$PADDLE_PRICE_STARTER
       PADDLE_PRICE_PRO=$PADDLE_PRICE_PRO
+      LITESTREAM_R2_BUCKET=$LITESTREAM_R2_BUCKET
+      LITESTREAM_R2_ACCESS_KEY_ID=$LITESTREAM_R2_ACCESS_KEY_ID
+      LITESTREAM_R2_SECRET_ACCESS_KEY=$LITESTREAM_R2_SECRET_ACCESS_KEY
+      LITESTREAM_R2_ENDPOINT=$LITESTREAM_R2_ENDPOINT
       ENVEOF
     - ssh "$DEPLOY_USER@$DEPLOY_HOST" "chmod 600 /opt/padelnomics/padelnomics/.env"
     - ssh "$DEPLOY_USER@$DEPLOY_HOST" "cd /opt/padelnomics && git pull origin master && ./deploy.sh"