Add multi-user support: Authelia OIDC, SQLite, Garage S3

- Auth via Authelia OIDC (public client + PKCE) with signed session cookies
- SQLite DB for users, rides, and insights (SQLAlchemy)
- Garage S3 for GPX file storage (boto3)
- All API endpoints scoped to authenticated user
- Dockerfile + docker-compose.yml with Garage service
- Frontend auth check: redirects to /auth/login on 401, shows user badge

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

.dockerignore

@@ -0,0 +1,9 @@
+.venv
+.env
+bikeslop.db
+garmin_gpx_exports/*.gpx
+insights.json
+__pycache__
+*.pyc
+*.pyo
+.git