feat: Create comments table

Closes #7 - created SQL comments table

server/db/schema/comments.sql

@@ -0,0 +1,11 @@
+CREATE TYPE adjustment AS ENUM ('none', 'total', 'rate_flat', 'rate_percent', 'paid');
+
+CREATE TABLE comments (
+    id INT PRIMARY KEY UNIQUE NOT NULL DEFAULT nextval('comment_id_seq'),
+    user_id INT REFERENCES users(id) NOT NULL,
+    invoice_id INT REFERENCES invoices(id) NOT NULL,
+    datetime TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP,
+    comment TEXT NOT NULL,
+    adjustment_type adjustment DEFAULT 'none',
+    adjustment_value NUMERIC DEFAULT 0
+);