Award credits for signups and referrals (500 signup, 250 per successful referral)

[afrinxnahar]

🎯 Problem Statement
Currently we are only adding 10 credits both for sign up and referral program which should be changed based on our new pricing tier.

💡 Proposed Solution

• On signup (email verified), credit user 500.
• Add a small message during sign up about the awarded credit.
• Upon successful referrral, award 250 credits to both referrer and referee.

📊 Impact Assessment
User Impact: High.
Development Effort: Low–Medium.
Priority: Medium–High.

🧪 Acceptance Criteria

• New users receive 500 credits.
• Referrer receives 250 credits when referral condition met.
• Transactions auditable.

🔗 Related Features
Credit system, signup flows, notifications, marketing.