Keep test helpers out of the shipped binary (closes #506)

The test helpers lived in ordinary `testing.go` files inside the config, database, middleware and session packages, so they were built into the binary and the shared `test-support` lint rule could not see them. The four files are gone: the session's helpers move into its own `_test.go` file, and the rest into `configtest`, `databasetest` and `middlewaretest`, which the `depguard` deny list now names, so a non-test file importing them fails lint. The test-support packages build through the production constructors.

Judgement call: the session, the middleware and the webhook database manager now take the plain logger they log through, which the application wiring provides.
Judgement call: two idle-expiry tests move the stored timestamps back instead of advancing a fake clock.

Model: opus-5-5

internal/gormlog/firstboot_test.go

@@ -3,6 +3,7 @@ package gormlog_test
 import (
 	"context"
 	"database/sql"
+	"log/slog"
 	"os"
 	"path/filepath"
 	"testing"
@@ -13,6 +14,7 @@ import (
 	"go.uber.org/fx/fxtest"
 	_ "modernc.org/sqlite" // Pure Go SQLite driver.
 	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/config/configtest"
 	"sneak.berlin/go/webhooker/internal/database"
 	"sneak.berlin/go/webhooker/internal/globals"
 	"sneak.berlin/go/webhooker/internal/logger"
@@ -128,7 +130,7 @@ func readFirstBootSecrets(
 func bootAtDebug(t *testing.T, dataDir string) string {
 	t.Helper()
 
-	config.ClearEnvForTest(t)
+	configtest.ClearEnv(t)
 	t.Setenv("DEBUG", "true")
 	t.Setenv("DATA_DIR", dataDir)
 
@@ -145,6 +147,7 @@ func bootAtDebug(t *testing.T, dataDir string) string {
 		fx.Provide(
 			globals.New,
 			logger.New,
+			func(l *logger.Logger) *slog.Logger { return l.Get() },
 			config.New,
 			database.New,
 			session.New,