Move the test-only helpers out of the shipped db package (closes #248)

The server binary carried exported names that only tests call, including
raw SQL access and a way to weaken the password hash.

Delete testing.go, GetDB and SetBcryptCost from the shipped code, so
passwords always hash at bcrypt.DefaultCost. Move GetDB, GetOrCreateChannel,
JoinChannel and HasChannelInvite into export_test.go. Delete
GetClientHostInfo, whose remaining check GetLatestClientForSession covers.

Tests in other packages now build the database with db.New and run raw SQL
through dbtest.Exec.

Model: opus-5-5

internal/db/db.go

@@ -85,11 +85,6 @@ func New(
 	return database, nil
 }
 
-// GetDB returns the underlying sql.DB connection.
-func (database *Database) GetDB() *sql.DB {
-	return database.conn
-}
-
 func (database *Database) connect(ctx context.Context) error {
 	dbURL := database.params.Config.DBURL
 	if dbURL == "" {