feat: implement subscription webhooks

[shaaibu7]

Summary

Implemented subscription webhook support across the contract, backend delivery layer, and React Native app. Merchants can now
register webhook endpoints for filtered subscription lifecycle events, inspect delivery history, pause/resume webhooks, retry
failed deliveries, and view basic health/analytics.

What changed

• Added shared webhook types for configs, retry policy, event payloads, delivery status, and analytics.
• Extended the Soroban subscription contract with webhook storage and lifecycle hooks.
• Added webhook event snapshots for subscription state and plan state.
• Added backend webhook delivery service with:
  • HMAC signature generation and verification
  • configurable exponential backoff retries
  • max retry handling
  • payload size guard
  • idempotency handling
  • health checks and analytics
• Added React Native webhook management UI and persisted local webhook state.
• Wired webhook settings into app navigation and settings.

Verification

• Added backend tests for signature verification, retry behavior, payload size limits, and manual retry.
• Attempted contract compilation, but local verification is currently blocked by the existing contracts workspace manifest
  resolution issue in this environment.
• Attempted Jest execution, but jest is not installed in the workspace.

Notes

• The implementation follows the issue requirements for registration, event filtering, delivery logging, manual retry, pausing,
  health checks, and analytics.
• Contract-side delivery records are persisted, while actual HTTP delivery/retry logic lives in the backend service where network
  I/O is possible.

closes #174

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