Add dispute events, subscription engine, batch verification, and relative stream creation`

[miss-yusrah]

closes #103
closes #107
closes #112
closes #126

Pr Description

Task 1 - Standardized Event Emissions for Dispute Lifecycle:

• Updated event names from DISPUTE/OPENED → DISPUTE_CREATED, DISPUTE/UNDER_REVIEW → DISPUTE_REVIEWED, DISPUTE/RESOLVED → DISPUTE_RESOLVED, DISPUTE/REJECTED → DISPUTE_REJECTED
• Events now include dispute_id and payment_id as specified
• Updated EVENTS.md documentation

Task 2 - Subscription/Recurring Payment Engine:

• Added Subscription and SubscriptionPlan structs
• Added SubscriptionStatus enum (Active, Paused, Cancelled, Expired)
• Added storage keys for subscription plans and subscriptions
• Implemented functions: create_subscription_plan, subscribe, pause_subscription, resume_subscription, cancel_subscription, process_due_subscriptions

Task 3 - Batch Payment Verification:

• Added verify_batch function to PaymentLinkManager that takes a Vec<String> of link IDs and returns payment status for each

Task 4 - create_stream_relative Entry-Point:

• Added Stream and StreamStatus structs
• Implemented create_stream_relative function that accepts relative time offsets (start_delay, cliff_delay, duration) instead of absolute timestamps
• Validates cliff_delay >= start_delay and duration > 0
• Computes absolute times using env.ledger().timestamp()

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