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/handlers/handlers_test.go

@@ -55,9 +55,10 @@ func setupTestHandler(t *testing.T) *testFixtures {
 	}
 
 	svc, err := imgcache.NewService(&imgcache.ServiceConfig{
-		Cache:     cache,
-		Fetcher:   newMockFetcher(mockFS),
-		Whitelist: []string{goodHost},
+		Cache:      cache,
+		Fetcher:    newMockFetcher(mockFS),
+		SigningKey: "test-signing-key-must-be-32-chars",
+		Whitelist:  []string{goodHost},
 	})
 	if err != nil {
 		t.Fatalf("failed to create service: %v", err)