fix: correct BC Admin Center API endpoints for database export

- Fix endpoint paths from /applications/businesscentral/environments/{env}/databaseExports
  to /exports/applications/BusinessCentral/environments/{env}
- Add Azure Storage SAS URI support (required by current export API)
- Update default API version from v2.15 to v2.21
- Add export metrics check before initiating export
- Use export history endpoint for status polling
- Download BACPAC from Azure Storage instead of expecting blobUri response

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

bc-backup.sh

@@ -39,6 +39,7 @@ required_vars=(
     "AZURE_CLIENT_ID"
     "AZURE_CLIENT_SECRET"
     "BC_ENVIRONMENT_NAME"
+    "AZURE_STORAGE_SAS_URI"
     "ENCRYPTION_PASSPHRASE"
     "S3_BUCKET"
     "S3_ENDPOINT"
@@ -58,7 +59,7 @@ RETENTION_DAYS="${RETENTION_DAYS:-30}"
 S3_TOOL="${S3_TOOL:-awscli}"
 MAX_RETRIES="${MAX_RETRIES:-3}"
 CLEANUP_LOCAL="${CLEANUP_LOCAL:-true}"
-BC_API_VERSION="${BC_API_VERSION:-v2.15}"
+BC_API_VERSION="${BC_API_VERSION:-v2.21}"
 
 log "========================================="
 log "Starting Business Central backup process"
@@ -79,6 +80,8 @@ export AZURE_CLIENT_ID
 export AZURE_CLIENT_SECRET
 export BC_ENVIRONMENT_NAME
 export BC_API_VERSION
+export AZURE_STORAGE_SAS_URI
+export AZURE_STORAGE_CONTAINER
 export WORK_DIR
 
 BACPAC_FILE="${WORK_DIR}/${BACKUP_FILENAME}.bacpac"