fix(migrations): change job_payments FK types from UUID to INT

- job_id changed from UUID to INT to match jobs.id SERIAL
- user_id changed from UUID to INT to match users.id SERIAL
- Added user_id FK to users table

backend/migrations/019_create_job_payments_table.sql

@@ -3,8 +3,8 @@
 
 CREATE TABLE IF NOT EXISTS job_payments (
     id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
-    job_id UUID NOT NULL,
-    user_id UUID,
+    job_id INT NOT NULL,
+    user_id INT,
     
     -- Stripe
     stripe_session_id VARCHAR(255),
@@ -39,8 +39,9 @@ CREATE TABLE IF NOT EXISTS job_payments (
     paid_at TIMESTAMP,
     expires_at TIMESTAMP,
     
-    -- Foreign key (UUID to match jobs table)
-    FOREIGN KEY (job_id) REFERENCES jobs(id) ON DELETE CASCADE
+    -- Foreign key (INT to match jobs.id SERIAL)
+    FOREIGN KEY (job_id) REFERENCES jobs(id) ON DELETE CASCADE,
+    FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE SET NULL
 );
 
 -- Indexes