feat: add interactive API tutorial for new developers

[elijah4196]

Add Interactive API Tutorial for New Developers

Summary

Adds a step-by-step Postman collection that walks new developers through the complete ProxyPay developer flow in under 30 minutes: register → KYC → deposit → withdraw.

What's included

New files in postman/

File	Description
developer_tutorial.postman_collection.json	Main tutorial collection with 4 ordered steps + bonus section
developer_tutorial.postman_environment.json	Pre-configured environment with all tutorial variables

Tutorial structure

Step 1 — Register & Authenticate (~5 min)

• 1.1 Login with phone number → JWT token auto-saved to environment
• 1.2 Verify token is valid
• 1.3 Get current user profile → userId + stellarAddress auto-saved

Step 2 — KYC Verification (~8 min)

• 2.1 Create KYC applicant → applicantId auto-saved
• 2.2 Upload identity document (base64)
• 2.3 Start verification workflow → workflowRunId auto-saved
• 2.4 Poll KYC status until basic level confirmed

Step 3 — Deposit (~5 min)

• 3.1 Deposit 500 XAF via MTN mobile money → transactionId auto-saved
• 3.2 Poll transaction status until completed

Step 4 — Withdraw (~5 min)

• 4.1 Withdraw 200 XAF to mobile money → withdrawTransactionId auto-saved
• 4.2 Confirm withdrawal completed

Bonus — Transaction History

• List all transactions with pagination

How the tutorial meets acceptance criteria

• Runnable: Every request uses {{variables}} populated by the previous step's Tests script — zero manual copy-pasting required
• Commented: Every request has a description field explaining what it does, why, and what to expect; inline comments in request bodies explain each field
• < 30 minutes: 9 core requests across a linear flow; estimated 20 minutes including reading time
• Environment variables auto-flow: Login → token, profile → userId/stellarAddress, KYC → applicantId, transactions → transactionId — no friction between steps
• Global test guards: A collection-level test blocks on any 5xx error; per-step tests validate status codes and log helpful console messages

Testing

1. Start the local server: npm run dev
2. Import developer_tutorial.postman_collection.json into Postman
3. Import developer_tutorial.postman_environment.json and select it as the active environment
4. Run the collection in order using the Postman Runner or manually step by step
5. All Tests should pass in the sandbox environment

closes #272

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