db: share SQLite memory databases across connections

[yperbasis]

The default SQLite connection string, :memory:?cache=shared, opens a separate in-memory database for each pooled connection. Concurrent reads and writes can then fail with no such table: task_results or task_logs, even after migrations succeed. This caused an Erigon Kurtosis CI failure.

Treat bare :memory: as shared memory and normalize in-memory connection strings with query parameters to SQLite's file: URI form. Append the WAL option with & when a query is already present, so existing parameters remain valid. This fixes the current default and existing configurations that use it.

The regression test holds one connection open and reads a saved task log through a second connection. Before the fix, it failed with no such table: task_logs. After the fix, it passes for bare and default memory strings, standard and named memory URIs, and a database file.