Every PromptDatabase() (nodes, image monitor, API, diagnostics) re-ran
table creation and all migrations. Schema init is now cached per
normalised path (class-level set under a lock) and re-run only when the
file disappeared, after a restore, or after reset_schema_cache().
The copy-and-swap migrations drop a stale *_new table left by an
interrupted run, execute inside an explicit transaction with
PRAGMA foreign_keys = OFF (DROP TABLE prompts used to cascade-delete
every generated_images row), roll back cleanly and log and re-raise
sqlite3.Error instead of swallowing it.