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/service/service_test.go

@@ -9,7 +9,6 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"os"
 	"slices"
 	"strings"
 	"sync"
@@ -18,7 +17,6 @@ import (
 
 	"go.uber.org/fx"
 	"go.uber.org/fx/fxtest"
-	"golang.org/x/crypto/bcrypt"
 	"sneak.berlin/go/neoirc/internal/broker"
 	"sneak.berlin/go/neoirc/internal/config"
 	"sneak.berlin/go/neoirc/internal/db"
@@ -30,14 +28,10 @@ import (
 	"sneak.berlin/go/neoirc/pkg/irc"
 )
 
-func TestMain(m *testing.M) {
-	db.SetBcryptCost(bcrypt.MinCost)
-	os.Exit(m.Run())
-}
-
 // testEnv holds all dependencies for a service test.
 type testEnv struct {
 	svc    *service.Service
+	dbURL  string
 	db     *db.Database
 	broker *broker.Broker
 	app    *fxtest.App
@@ -111,6 +105,7 @@ func newTestEnv(t *testing.T) *testEnv {
 
 	return &testEnv{
 		svc:    svc,
+		dbURL:  dbURL,
 		db:     database,
 		broker: brk,
 		app:    app,
@@ -940,19 +935,16 @@ func TestSendDirectMessageTargetQuits(t *testing.T) {
 	sid := createSession(ctx, t, env.db, "alice")
 	createSession(ctx, t, env.db, "bob")
 
-	_, err := env.db.GetDB().ExecContext(ctx,
+	dbtest.Exec(t, env.dbURL,
 		`CREATE TRIGGER delete_recipient
 		 BEFORE INSERT ON messages
 		 BEGIN
 		   DELETE FROM sessions WHERE id = NEW.to_session_id;
 		 END`)
-	if err != nil {
-		t.Fatalf("create trigger: %v", err)
-	}
 
 	body, _ := json.Marshal([]string{"hi"}) //nolint:errchkjson
 
-	_, err = env.svc.SendDirectMessage(
+	_, err := env.svc.SendDirectMessage(
 		ctx, sid, 0, "alice", irc.CmdPrivmsg, "bob", body, nil,
 	)
 
@@ -978,15 +970,12 @@ func TestSendChannelMessageChannelDeleted(t *testing.T) {
 		t.Fatalf("join alice: %v", err)
 	}
 
-	_, err = env.db.GetDB().ExecContext(ctx,
+	dbtest.Exec(t, env.dbURL,
 		`CREATE TRIGGER delete_recipient
 		 BEFORE INSERT ON messages
 		 BEGIN
 		   DELETE FROM channels WHERE id = NEW.to_channel_id;
 		 END`)
-	if err != nil {
-		t.Fatalf("create trigger: %v", err)
-	}
 
 	body, _ := json.Marshal([]string{"hi"}) //nolint:errchkjson