feat: add file upload API endpoint, model, and repository method

- Create FileModels.kt with FileUploadResponse and UploadedFile data classes
- Add uploadFile() multipart endpoint to GhostApiService
- Add uploadFile(uri) method to PostRepository following the same pattern as uploadImage()

app/src/main/java/com/swoosh/microblog/data/api/GhostApiService.kt

@@ -1,5 +1,6 @@
 package com.swoosh.microblog.data.api
 
+import com.swoosh.microblog.data.model.FileUploadResponse
 import com.swoosh.microblog.data.model.GhostSite
 import com.swoosh.microblog.data.model.MembersResponse
 import com.swoosh.microblog.data.model.PageWrapper
@@ -114,6 +115,13 @@ interface GhostApiService {
         @Part file: MultipartBody.Part,
         @Part("purpose") purpose: RequestBody
     ): Response<ImageUploadResponse>
+
+    @Multipart
+    @POST("ghost/api/admin/files/upload/")
+    suspend fun uploadFile(
+        @Part file: MultipartBody.Part,
+        @Part("ref") ref: RequestBody? = null
+    ): Response<FileUploadResponse>
 }
 
 data class ImageUploadResponse(

app/src/main/java/com/swoosh/microblog/data/model/FileModels.kt

@@ -0,0 +1,10 @@
+package com.swoosh.microblog.data.model
+
+data class FileUploadResponse(
+    val files: List<UploadedFile>
+)
+
+data class UploadedFile(
+    val url: String,
+    val ref: String?
+)

app/src/main/java/com/swoosh/microblog/data/repository/PostRepository.kt

@@ -133,6 +133,28 @@ class PostRepository(private val context: Context) {
             }
         }
 
+    suspend fun uploadFile(uri: Uri): Result<String> =
+        withContext(Dispatchers.IO) {
+            try {
+                val file = copyUriToTempFile(uri)
+                val mimeType = context.contentResolver.getType(uri) ?: "application/octet-stream"
+                val requestBody = file.asRequestBody(mimeType.toMediaType())
+                val part = MultipartBody.Part.createFormData("file", file.name, requestBody)
+
+                val response = getApi().uploadFile(part)
+                file.delete()
+
+                if (response.isSuccessful) {
+                    val url = response.body()!!.files.first().url
+                    Result.success(url)
+                } else {
+                    Result.failure(Exception("File upload failed ${response.code()}"))
+                }
+            } catch (e: Exception) {
+                Result.failure(e)
+            }
+        }
+
     /**
      * Uploads multiple images and returns all uploaded URLs.
      * If any upload fails, returns failure with the error.