feat(dev): add synthetic event generator for local development (#699)

[ravendevhub]

Summary

Resolves #699 by introducing a development CLI utility (listener/src/scripts/generate-synthetic-events.ts) capable of synthesizing schema-compliant Soroban contract events for rapid local development and testing without live network latency or risk of accidental external notification dispatches.

Changes

1. Synthetic Event Generator Engine (listener/src/scripts/generate-synthetic-events.ts):
   • Generates representative events for transfer, task_created, and bounty_awarded event types.
   • Conforms to documented event schema with unique identifiers, timestamps, and ledger numbers.
   • Guarantees zero external network/webhook traffic by default.
2. Scripts & npm Targets:
   • Added npm run generate:events script to listener/package.json.
3. Tests & Documentation:
   • Automated unit test coverage in generate-synthetic-events.test.ts.
   • Comprehensive guide and CLI options documented in docs/SYNTHETIC_EVENT_GENERATOR.md.

Acceptance Criteria

• Developers can generate representative events locally.
• Generated data follows the documented event schema.
• The utility cannot accidentally send external notifications by default.
• Usage instructions are documented.