fix(testutil): use shared-cache SQLite so concurrent reads see same DB

SetupTestDB used `sqlite.Open(":memory:")`, which creates a *separate*
in-memory database for every connection in GORM's pool. Sequential tests
never noticed because the pool keeps reusing one connection — but the
moment any code path issued concurrent reads (e.g. errgroup-driven
parallel COUNT queries), a goroutine could pull a fresh connection, see
no migrated tables, and explode with "no such table".

Switched to `file:testdb_<n>?mode=memory&cache=shared&_journal=memory`
with a per-test atomic counter so every connection in the pool sees the
same in-memory DB and tests stay isolated from each other through the
unique cache namespace. As a bonus, this also resolves the pre-existing
TestTaskHandler_QuickComplete flake — same root cause, just intermittent
because the pool occasionally handed out a second connection.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/testutil/testutil.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"sync"
+	"sync/atomic"
 	"testing"
 
 	"github.com/labstack/echo/v4"
@@ -21,7 +22,10 @@ import (
 	"github.com/treytartt/honeydue-api/internal/validator"
 )
 
-var i18nOnce sync.Once
+var (
+	i18nOnce       sync.Once
+	testDBCounter  uint64
+)
 
 // SetupTestDB creates an in-memory SQLite database for testing
 func SetupTestDB(t *testing.T) *gorm.DB {
@@ -30,7 +34,16 @@ func SetupTestDB(t *testing.T) *gorm.DB {
 		i18n.Init()
 	})
 
-	db, err := gorm.Open(sqlite.Open(":memory:"), &gorm.Config{
+	// SQLite in-memory + GORM connection pool needs `cache=shared` so all
+	// pool connections see the same DB. With plain ":memory:", each new
+	// connection gets its own empty database, which makes errgroup-driven
+	// parallel reads explode with "no such table" once a goroutine pulls a
+	// fresh connection. The DSN is uniqued per test invocation (atomic
+	// counter) so tests don't bleed state into each other through the shared
+	// cache namespace.
+	id := atomic.AddUint64(&testDBCounter, 1)
+	dsn := fmt.Sprintf("file:testdb_%d?mode=memory&cache=shared&_journal=memory", id)
+	db, err := gorm.Open(sqlite.Open(dsn), &gorm.Config{
 		Logger: logger.Default.LogMode(logger.Silent),
 	})
 	require.NoError(t, err)