feat(feedback): Add content improvement feedback system

Frontend (frontend/app.js):

- Add textarea for improvement feedback

- Add submit button with loading state

- Handle API response and display improved content

Backend (backend/copywriter.py):

- Add improve_copy() method using Cohere API

- Integrate retry mechanism for API calls

Backend (backend/main.py):

- Add /improve-content POST endpoint

- Implement error handling and return improved content with metadata

Testing:

- Verified feedback submission flow

- Confirmed improved content generation

- Tested error scenarios and loading states

data/past_campaigns/email_20240101.json

@@ -0,0 +1,16 @@
+{
+  "content": "Subject: Ready to Transform Your Business? Take Action Now\n\nYou've been thinking about this moment.\n\nThe moment when you finally step into your full business potential.\n\nRight now, you're standing at a crossroads. One path leads to more of the same. The other leads to breakthrough results.\n\nThe Business Mastery Program opens next week. Inside, you'll discover:\n\n• Proven strategies that eliminate second-guessing\n• Clear systems for consistent growth\n• Powerful techniques to overcome any business challenge\n• Direct access to transformational tools\n\nMake the decision that your future self will thank you for.\n\nClick here to secure your spot → [CTA Button]\n\nP.S. Early registration closes in 48 hours. Claim your bonus strategy session now.",
+  "content_type": "email",
+  "metadata": {
+    "campaign_name": "Business Mastery Launch",
+    "performance_metrics": {
+      "open_rate": 0.42,
+      "click_rate": 0.15,
+      "conversion_rate": 0.08
+    },
+    "added_at": "2024-01-01T00:00:00Z",
+    "training_data": true
+  },
+  "document_id": 2,
+  "timestamp": "2024-01-01T00:00:00Z"
+}