fix: subscribe() was sending tokens out instead of collecting them

[PragyaTripathi990]

Summary

Critical logic bug: subscribe() called pptToken.transfer(msg.sender, SUBSCRIPTION_AMOUNT) which sent 10 PPT tokens to the subscriber for free instead of charging them. Anyone could call subscribe() and drain the contract's token balance.

Fixed by changing to pptToken.transferFrom(msg.sender, address(this), SUBSCRIPTION_AMOUNT) — the correct pull-payment pattern where the subscriber pays the fee.

Also added:

• An explicit allowance check with a human-readable error message so callers understand they need to approve() the contract before subscribing.
• A TokensWithdrawn event on withdrawTokens() for on-chain auditability.

Before / After

// BEFORE (buggy — sends tokens TO subscriber)
require(pptToken.transfer(msg.sender, SUBSCRIPTION_AMOUNT), "Token transfer failed");

// AFTER (correct — pulls tokens FROM subscriber)
require(
    pptToken.allowance(msg.sender, address(this)) >= SUBSCRIPTION_AMOUNT,
    "Insufficient token allowance. Approve the contract first."
);
require(
    pptToken.transferFrom(msg.sender, address(this), SUBSCRIPTION_AMOUNT),
    "Token transfer failed"
);

Test plan

• Deploy PPTToken + MedInvoiceContract locally
• Calling subscribe() without approving → reverts with allowance error
• Calling subscribe() after approve(contract, 10e18) → succeeds, deducts 10 PPT from caller
• Calling subscribe() twice → reverts with "Already subscribed"

Closes #1

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