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/auth.go

@@ -10,13 +10,6 @@ import (
 	"golang.org/x/crypto/bcrypt"
 )
 
-//nolint:gochecknoglobals // var so tests can override via SetBcryptCost
-var bcryptCost = bcrypt.DefaultCost
-
-// SetBcryptCost overrides the bcrypt cost.
-// Use bcrypt.MinCost in tests to avoid slow hashing.
-func SetBcryptCost(cost int) { bcryptCost = cost }
-
 var errNoPassword = errors.New(
 	"account has no password set",
 )
@@ -29,7 +22,7 @@ func (database *Database) SetPassword(
 	password string,
 ) error {
 	hash, err := bcrypt.GenerateFromPassword(
-		[]byte(password), bcryptCost,
+		[]byte(password), bcrypt.DefaultCost,
 	)
 	if err != nil {
 		return fmt.Errorf("hash password: %w", err)