feat: add plaintext admin message endpoint for testing

- New POST /api/admin/send-message endpoint (admin-only)
- Messages prefixed with [PLAINTEXT] bypass E2EE decryption
- App recognizes [PLAINTEXT] marker and strips it before display
- Allows easy chat testing without E2EE key management

app/src/main/java/de/bollwerk/app/data/repository/MessageRepositoryImpl.kt

@@ -70,11 +70,20 @@ internal class MessageRepositoryImpl @Inject constructor(
                 when (event) {
                     is WebSocketEvent.NewMessage -> {
                         val msg = event.message
-                        val decryptedBody = try {
-                            e2eeKeyManager.decryptMessage(msg.body)
-                        } catch (e: Exception) {
-                            Log.e(TAG, "E2EE: Failed to decrypt message ${msg.id}", e)
-                            "[Entschlüsselung fehlgeschlagen]"
+                        val decryptedBody = when {
+                            msg.body.startsWith("[PLAINTEXT] ") -> {
+                                // Admin test messages – remove prefix
+                                msg.body.removePrefix("[PLAINTEXT] ")
+                            }
+                            else -> {
+                                // E2EE messages – decrypt
+                                try {
+                                    e2eeKeyManager.decryptMessage(msg.body)
+                                } catch (e: Exception) {
+                                    Log.e(TAG, "E2EE: Failed to decrypt message ${msg.id}", e)
+                                    "[Entschlüsselung fehlgeschlagen]"
+                                }
+                            }
                         }
                         dao.upsert(
                             MessageEntity(

server/src/main/kotlin/de/bollwerk/server/routes/MessageRoutes.kt

@@ -95,4 +95,54 @@ internal fun Route.messageRoutes(
             call.respond(HttpStatusCode.OK, messages)
         }
     }
+
+    route("/api/admin/send-message") {
+        post {
+            val principal = call.principal<UserPrincipal>()
+                ?: return@post call.respond(
+                    HttpStatusCode.Unauthorized,
+                    ErrorResponse(status = 401, message = "Unauthorized")
+                )
+            // Only admins can send test messages
+            if (!principal.isAdmin) {
+                return@post call.respond(
+                    HttpStatusCode.Forbidden,
+                    ErrorResponse(status = 403, message = "Admin access required")
+                )
+            }
+
+            val request = call.receive<SendMessageRequest>()
+            if (request.body.isBlank()) {
+                call.respond(
+                    HttpStatusCode.BadRequest,
+                    ErrorResponse(status = 400, message = "Body must not be empty")
+                )
+                return@post
+            }
+            if (userRepository.findById(request.receiverId) == null) {
+                call.respond(
+                    HttpStatusCode.NotFound,
+                    ErrorResponse(status = 404, message = "Receiver not found")
+                )
+                return@post
+            }
+
+            val msgId = request.id ?: UUID.randomUUID().toString()
+            // Wrap body with [PLAINTEXT] marker so app doesn't try to decrypt
+            val plaintext = "[PLAINTEXT] ${request.body}"
+            val message = messageRepository.save(
+                id = msgId,
+                senderId = principal.userId,
+                senderUsername = principal.username,
+                receiverId = request.receiverId,
+                body = plaintext,
+                sentAt = request.sentAt
+            )
+            wsManager.notifyNewMessage(request.receiverId, message)
+            if (wsManager.isOnline(request.receiverId)) {
+                messageRepository.markDelivered(msgId)
+            }
+            call.respond(HttpStatusCode.Created, SendMessageResponse(message = message))
+        }
+    }
 }