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/session/session.go

@@ -16,7 +16,6 @@ import (
 	"go.uber.org/fx"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/database"
-	"sneak.berlin/go/webhooker/internal/logger"
 	"sneak.berlin/go/webhooker/internal/reqtls"
 )
 
@@ -80,7 +79,7 @@ type Params struct {
 
 	Config   *config.Config
 	Database *database.Database
-	Logger   *logger.Logger
+	Logger   *slog.Logger
 }
 
 // Session manages encrypted session storage.
@@ -180,7 +179,7 @@ func New(
 	params Params,
 ) (*Session, error) {
 	s := &Session{
-		log:         params.Logger.Get(),
+		log:         params.Logger,
 		idleTimeout: params.Config.SessionIdleTimeout,
 		now:         time.Now,
 	}