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/database/retention_test.go

@@ -51,7 +51,7 @@ func setupRetentionTest(t *testing.T) *retentionTestEnv {
 
 	mgr, err := database.NewWebhookDBManager(
 		lc,
-		database.WebhookDBManagerParams{Config: cfg, Logger: l},
+		database.WebhookDBManagerParams{Config: cfg, Logger: l.Get()},
 	)
 	require.NoError(t, err)