fix: assorted ActivityPub code only refactors (#8274)

Fix parts of issue https://codeberg.org/forgejo/forgejo/issues/8221
and PR https://codeberg.org/forgejo/forgejo/pulls/4767

- PostgreSQL
- TestActivityPubPerson/SignedRequestValidation

```
    --- FAIL: TestActivityPubPerson/SignedRequestValidation (5.01s)
        api_activitypub_person_test.go:51:
            	Error Trace:	/workspace/forgejo/forgejo/tests/integration/api_activitypub_person_test.go:51
            	Error:      	Received unexpected error:
            	            	Get "http://127.0.0.1:3002/api/v1/activitypub/user-id/2": context deadline exceeded (Client.Timeout exceeded while awaiting headers)
            	Test:       	TestActivityPubPerson/SignedRequestValidation
    testlogger.go:411: 2025/06/24 00:12:27 ...eb/routing/logger.go:102:func1() [I] router: completed GET /api/v1/activitypub/user-id/2 for 127.0.0.1:50456, 200 OK in 5032.2ms @ activitypub/person.go:21(activitypub.Person)
```

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8274
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Michael Jerger <michael.jerger@meissa-gmbh.de>
Co-committed-by: Michael Jerger <michael.jerger@meissa-gmbh.de>

services/federation/error.go

@@ -0,0 +1,44 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package federation
+
+import (
+	"fmt"
+	"net/http"
+)
+
+type ErrNotAcceptable struct {
+	Message string
+}
+
+func NewErrNotAcceptablef(format string, a ...any) ErrNotAcceptable {
+	message := fmt.Sprintf(format, a...)
+	return ErrNotAcceptable{Message: message}
+}
+
+func (err ErrNotAcceptable) Error() string {
+	return fmt.Sprintf("NotAcceptable: %v", err.Message)
+}
+
+type ErrInternal struct {
+	Message string
+}
+
+func NewErrInternalf(format string, a ...any) ErrInternal {
+	message := fmt.Sprintf(format, a...)
+	return ErrInternal{Message: message}
+}
+
+func (err ErrInternal) Error() string {
+	return fmt.Sprintf("InternalServerError: %v", err.Message)
+}
+
+func HTTPStatus(err error) int {
+	switch err.(type) {
+	case ErrNotAcceptable:
+		return http.StatusNotAcceptable
+	default:
+		return http.StatusInternalServerError
+	}
+}