feat(backup): add GCS offsite backup executor

Add gcs_backup_executor with git_bundle mode for backing up bare git
repos to Google Cloud Storage. Includes retention management and
bundle verification. Adds google-cloud-storage dependency and
GCS_CREDENTIALS_FILE setting.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "the-scheduler"
-version = "1.1.3"
+version = "1.2.0"
 description = "System-wide maintenance orchestration - backups, doc mirroring, cleanup, task automation"
 readme = "README.md"
 requires-python = ">=3.12"
@@ -26,6 +26,8 @@ dependencies = [
     "gitpython~=3.1.43",
     # Security/Auth
     "python-jose[cryptography]~=3.3.0",
+    # Cloud storage
+    "google-cloud-storage~=2.18.0",
 ]
 
 [project.optional-dependencies]