feat(frontend): add storage service for S3 file uploads

- Create storage.ts with pre-signed URL handling
- Implement getUploadUrl, uploadFileToS3 helper functions
- Add complete uploadFile workflow for easy file uploads
- Support logos, resumes, documents, avatars folders

frontend/src/lib/storage.ts

@@ -0,0 +1,143 @@
+/**
+ * Storage service for S3 file uploads via pre-signed URLs
+ */
+
+const API_URL = process.env.NEXT_PUBLIC_API_URL || "http://localhost:8080/api/v1";
+
+interface UploadUrlResponse {
+    uploadUrl: string;
+    key: string;
+    publicUrl: string;
+    expiresIn: number;
+}
+
+interface DownloadUrlResponse {
+    downloadUrl: string;
+    expiresIn: number;
+}
+
+/**
+ * Get a pre-signed URL for uploading a file
+ */
+export async function getUploadUrl(
+    filename: string,
+    contentType: string,
+    folder: 'logos' | 'resumes' | 'documents' | 'avatars' = 'documents'
+): Promise<UploadUrlResponse> {
+    const token = typeof window !== 'undefined' ? localStorage.getItem('auth_token') : null;
+
+    if (!token) {
+        throw new Error('Not authenticated');
+    }
+
+    const response = await fetch(`${API_URL}/storage/upload-url`, {
+        method: 'POST',
+        headers: {
+            'Content-Type': 'application/json',
+            'Authorization': `Bearer ${token}`,
+        },
+        body: JSON.stringify({
+            filename,
+            contentType,
+            folder,
+        }),
+    });
+
+    if (!response.ok) {
+        const error = await response.text();
+        throw new Error(`Failed to get upload URL: ${error}`);
+    }
+
+    return response.json();
+}
+
+/**
+ * Upload a file directly to S3 using a pre-signed URL
+ */
+export async function uploadFileToS3(
+    file: File,
+    uploadUrl: string
+): Promise<void> {
+    const response = await fetch(uploadUrl, {
+        method: 'PUT',
+        headers: {
+            'Content-Type': file.type,
+        },
+        body: file,
+    });
+
+    if (!response.ok) {
+        throw new Error(`Failed to upload file: ${response.statusText}`);
+    }
+}
+
+/**
+ * Get a pre-signed URL for downloading a file
+ */
+export async function getDownloadUrl(key: string): Promise<DownloadUrlResponse> {
+    const token = typeof window !== 'undefined' ? localStorage.getItem('auth_token') : null;
+
+    if (!token) {
+        throw new Error('Not authenticated');
+    }
+
+    const response = await fetch(`${API_URL}/storage/download-url`, {
+        method: 'POST',
+        headers: {
+            'Content-Type': 'application/json',
+            'Authorization': `Bearer ${token}`,
+        },
+        body: JSON.stringify({ key }),
+    });
+
+    if (!response.ok) {
+        const error = await response.text();
+        throw new Error(`Failed to get download URL: ${error}`);
+    }
+
+    return response.json();
+}
+
+/**
+ * Complete file upload workflow: get pre-signed URL and upload file
+ * Returns the public URL of the uploaded file
+ */
+export async function uploadFile(
+    file: File,
+    folder: 'logos' | 'resumes' | 'documents' | 'avatars' = 'documents'
+): Promise<{ key: string; publicUrl: string }> {
+    // Step 1: Get pre-signed upload URL from backend
+    const { uploadUrl, key, publicUrl } = await getUploadUrl(
+        file.name,
+        file.type,
+        folder
+    );
+
+    // Step 2: Upload file directly to S3
+    await uploadFileToS3(file, uploadUrl);
+
+    return { key, publicUrl };
+}
+
+/**
+ * Delete a file from storage
+ */
+export async function deleteFile(key: string): Promise<void> {
+    const token = typeof window !== 'undefined' ? localStorage.getItem('auth_token') : null;
+
+    if (!token) {
+        throw new Error('Not authenticated');
+    }
+
+    const response = await fetch(`${API_URL}/storage/files?key=${encodeURIComponent(key)}`, {
+        method: 'DELETE',
+        headers: {
+            'Authorization': `Bearer ${token}`,
+        },
+    });
+
+    if (!response.ok) {
+        const error = await response.text();
+        throw new Error(`Failed to delete file: ${error}`);
+    }
+}