StoreAndForwardStorage.InitializeAsync opened a SqliteConnection against the
configured SqliteDbPath (default ./data/store-and-forward.db) without ensuring
the parent directory exists. SQLite creates the database file but not its
directory, so when data/ was absent the connection failed with
"SQLite Error 14: unable to open database file" — aborting the site host's
RegisterSiteActors at StoreAndForwardService.StartAsync.
This was the root cause of the six failing SiteActorPathTests. Production
masked it because the Docker image / deployment creates data/.
InitializeAsync now calls EnsureDatabaseDirectoryExists, which parses the
connection string and creates the parent directory of a file-backed database
(in-memory databases and bare filenames are skipped).
Regression test InitializeAsync_FileInMissingDirectory_CreatesDirectory fails
against the pre-fix code. Host suite now 155/155 green (was 149/155).
