fix: Update all pipelines to Python 3.14

Python 3.13 is causing installation errors in CI/CD runners.
Upgrading to Python 3.14 which is the latest stable version.

Changes:
- pyproject.toml: Update python requirement to ^3.14
- pyproject.toml: Update MyPy python_version to 3.14
- pyproject.toml: Update Black target-version to py314
- .gitlab-ci.yml: Update PYTHON_VERSION to 3.14
- .github/workflows/build-deploy.yml: Update PYTHON_VERSION to 3.14
- .gitea/workflows/ci.yml: Update PYTHON_VERSION to 3.14

This fixes the CI/CD error:
"rm: cannot remove '/opt/hostedtoolcache/Python/3.13.9/x64/lib/python3.13/__pycache__': Directory not empty"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.github/workflows/build-deploy.yml

@@ -26,7 +26,7 @@ on:
 env:
   DOCKER_REGISTRY: ghcr.io
   IMAGE_NAME: ${{ github.repository }}/docs-server
-  PYTHON_VERSION: '3.13'
+  PYTHON_VERSION: '3.14'
 
 jobs:
   # Job 1: Linting e validazione