chore: remove .gitlab-ci.yml (GitLab now backup-only mirror)

CI runs on Gitea only. GitLab is a passive push mirror — no runners,
no tagging, no deploy involvement.

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

.gitlab-ci.yml

@@ -1,31 +0,0 @@
-stages:
-  - test
-  - tag
-
-test:
-  stage: test
-  image: python:3.12-slim
-  before_script:
-    - pip install uv
-  script:
-    - uv sync
-    - uv run pytest web/tests/ -x -q -p no:faulthandler
-    - uv run ruff check web/src/ web/tests/
-  rules:
-    - if: $CI_COMMIT_BRANCH == "master"
-    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
-
-tag:
-  stage: tag
-  image:
-    name: alpine/git
-    entrypoint: [""]
-  script:
-    - git tag "v${CI_PIPELINE_IID}"
-    - git push "https://gitlab-ci-token:${CI_JOB_TOKEN}@${CI_SERVER_HOST}/${CI_PROJECT_PATH}.git" "v${CI_PIPELINE_IID}"
-  rules:
-    - if: $CI_COMMIT_BRANCH == "master"
-
-# Deployment is handled by the on-server supervisor (src/padelnomics/supervisor.py).
-# It polls git every 60s, fetches tags, and deploys only when a new passing tag exists.
-# No CI secrets needed — zero SSH keys, zero deploy credentials.