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>

src/config.py

@@ -66,6 +66,9 @@ class Settings(BaseSettings):
     backup_retention_weekly: int = Field(default=4, alias="BACKUP_RETENTION_WEEKLY")
     backup_retention_monthly: int = Field(default=12, alias="BACKUP_RETENTION_MONTHLY")
 
+    # Google Cloud Storage
+    gcs_credentials_file: str = Field(default="", alias="GCS_CREDENTIALS_FILE")
+
     # Documentation Mirroring
     docs_mirror_path: str = Field(default="/docs-mirror", alias="DOCS_MIRROR_PATH")
     docs_check_interval: int = Field(default=21600, alias="DOCS_CHECK_INTERVAL")  # 6 hours