Add guest signature upload/download

src/main/kotlin/com/android/trisolarisserver/component/GuestSignatureStorage.kt

@@ -0,0 +1,44 @@
+package com.android.trisolarisserver.component
+
+import org.springframework.beans.factory.annotation.Value
+import org.springframework.stereotype.Component
+import org.springframework.web.multipart.MultipartFile
+import java.nio.file.Files
+import java.nio.file.Path
+import java.nio.file.Paths
+import java.util.UUID
+
+data class StoredSignature(
+    val storagePath: String,
+    val sizeBytes: Long
+)
+
+@Component
+class GuestSignatureStorage(
+    @Value("\${storage.documents.root}") private val root: String
+) {
+    init {
+        val rootPath = Paths.get(root)
+        Files.createDirectories(rootPath)
+        if (!Files.isWritable(rootPath)) {
+            throw IllegalStateException("Guest signature root not writable: $root")
+        }
+    }
+
+    fun store(propertyId: UUID, guestId: UUID, file: MultipartFile): StoredSignature {
+        val dir = Paths.get(root, "guests", propertyId.toString(), guestId.toString())
+        Files.createDirectories(dir)
+        val path = dir.resolve("signature.svg")
+        file.inputStream.use { input ->
+            Files.newOutputStream(path).use { output -> input.copyTo(output) }
+        }
+        return StoredSignature(
+            storagePath = path.toString(),
+            sizeBytes = Files.size(path)
+        )
+    }
+
+    fun resolvePath(storagePath: String): Path {
+        return Paths.get(storagePath)
+    }
+}