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. Handler tests that need a password only to log in store
one hashed at bcrypt.MinCost. Tests of PASS itself keep the real cost.

Model: opus-5-5

internal/db/queries_test.go

@@ -125,7 +125,7 @@ func TestCreateSessionStoresIP(t *testing.T) {
 	database := setupTestDB(t)
 	ctx := t.Context()
 
-	sessionID, clientID, _, err := database.CreateSession(
+	sessionID, _, _, err := database.CreateSession(
 		ctx, "ipuser", "ident", "host.example.com",
 		"192.168.1.42",
 	)
@@ -147,8 +147,8 @@ func TestCreateSessionStoresIP(t *testing.T) {
 		)
 	}
 
-	clientInfo, err := database.GetClientHostInfo(
-		ctx, clientID,
+	clientInfo, err := database.GetLatestClientForSession(
+		ctx, sessionID,
 	)
 	if err != nil {
 		t.Fatal(err)
@@ -169,19 +169,6 @@ func TestCreateSessionStoresIP(t *testing.T) {
 	}
 }
 
-func TestGetClientHostInfoNotFound(t *testing.T) {
-	t.Parallel()
-
-	database := setupTestDB(t)
-
-	_, err := database.GetClientHostInfo(
-		t.Context(), 99999,
-	)
-	if err == nil {
-		t.Fatal("expected error for nonexistent client")
-	}
-}
-
 func TestGetSessionHostInfoNotFound(t *testing.T) {
 	t.Parallel()