Remove runtime nil checks for always-initialized components

Since signing_key is now required at config load time, sessMgr, encGen,
and signer are always initialized. Remove unnecessary nil checks that
were runtime failure paths that can no longer be reached.

- handlers.go: Remove conditional init, always create sessMgr/encGen
- auth.go: Remove nil checks for sessMgr
- imageenc.go: Remove nil check for encGen
- service.go: Require signing_key in NewService, remove signer nil checks
- Update tests to provide signing_key

internal/imgcache/testutil_test.go

@@ -147,7 +147,7 @@ func SetupTestService(t *testing.T, opts ...TestServiceOption) (*Service, *TestF
 
 	cfg := &testServiceConfig{
 		whitelist:  []string{fixtures.GoodHost},
-		signingKey: "",
+		signingKey: "test-signing-key-must-be-32-chars",
 	}
 
 	for _, opt := range opts {