feat(newsletter): add newsletters listing and detail pages

* Created a newsletters table and added insertion logic during newsletter send.
* Added a /newsletters route to display a list of sent newsletters.
* Implemented a /newsletter/<int:newsletter_id> route for detailed view.
* Developed new templates with navigation, including unsubscribe links.
* Enhanced front-end styling and layout for newsletter pages.

database.py

@@ -25,6 +25,16 @@ def init_db():
             email TEXT UNIQUE NOT NULL
         )
     """)
+    
+    cursor.execute("""
+        CREATE TABLE IF NOT EXISTS newsletters(
+            id SERIAL PRIMARY KEY,
+            subject TEXT NOT NULL,
+            body TEXT NOT NULL,
+            sent_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+        )
+    """)
+        
     conn.commit()
     cursor.close()
     conn.close()