feat: emit contract events for create_prompt, buy_prompt, and price updates (#406)

[opratem]

Description

Implements structured Soroban event emissions and off-chain wiring for core marketplace actions so indexers, webhooks, and the frontend can react in real time instead of relying solely on polling get_all_prompts. The change adds explicit contract event coverage for prompt creation, purchases, and price updates, and connects those events to the existing webhook dispatcher and email notification services.

Related Issue

Closes #406

Type of Change

New feature
Documentation update

Changes Made

Added contract-level event assertions in `contracts/prompt-hash/src/test.rs` to verify that `create_prompt` emits `PromptCreated` and `buy_prompt` emits `PromptPurchased`.
Extended `server/src/controllers/webhookControllers.ts` allowed event list to include `PromptCreated` and `PromptPriceUpdated` so creators can subscribe to newly created listings and price changes.
Wired `server/src/services/indexer.ts` to dispatch webhooks and send purchase notifications for `PromptCreated`, `PromptPurchased`, and `PromptPriceUpdated` events decoded from Soroban RPC streams.
Preserved existing indexer database behavior and added fail-open error logging so webhook/email failures never block the indexing loop.

Testing

Added Rust unit tests covering:
    `PromptCreated` emission on `create_prompt`.
    `PromptPurchased` emission on `buy_prompt`.
Verified that existing event coverage for price updates and sale-status changes remains intact.
Confirmed TypeScript source compiles without new type errors in the modified server files.

Checklist

Code follows Soroban SDK and server-side TypeScript best practices
Self-reviewed my code
Added comprehensive unit tests
Updated documentation where applicable
Proper error handling and non-blocking side effects in the indexer
No breaking changes to existing contract APIs or indexer database flow

Screenshots

N/A � Smart contract and server service changes only.

Additional Notes

The event emissions already existed on-chain for these actions; this work closes the gap by ensuring they are explicitly tested and that the off-chain indexer reacts to them. The frontend useContractSync hook already polls all contract events, so no client-side changes are required for real-time invalidation to begin using the new events.

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