[rbev]

Fail startup before ingestion when a database has pending migrations or the configured file system body storage directory is missing or unwritable.

• Add IStartupCheck and a hosted service that runs checks sequentially in an async scope before persistence and ingestion services start.
• Check pending migrations for SQL Server and PostgreSQL in both Main and Audit instances. Report unapplied migrations with instructions to run --setup.
• Move the body storage write probe from installation to runtime startup. The installer only creates the directory.
• Keep checks out of setup hosts, so --setup-and-run completes provisioning before validation runs. Runtime checks neither apply migrations nor create directories.
• Document each check and its prerequisites.