fix(indexer): align tradeParser's topic with the contract event convention

[melopaul]

Summary

• Every event currently published by contracts/market/src/events.rs follows the same pattern: Soroban's #[contractevent] macro snake-cases the event struct name including its literal Event suffix — MarketCreatedEvent → market_created_event, MarketResolvedEvent → market_resolved_event, CollateralDepositedEvent → collateral_deposited_event, etc. (confirmed against each event's own unit test in events.rs). tradeParser.ts's "trade_executed" discriminator was the one outlier still guessing the unsuffixed form — it's now "trade_executed_event", consistent with every other parser in this module after Fix resolutionParser for u32 bool u64 payload #589 and Wire market_created parser into ingestion loop #590.
• Unlike Fix resolutionParser for u32 bool u64 payload #589/Wire market_created parser into ingestion loop #590, this isn't fixing a live ingestion bug: the contract doesn't publish a trade execution event yet (trades are currently matched off-chain by the CLOB — see the Trade/IndexedTrade Prisma models). This aligns the parser with the established naming convention now, so that whenever TradeExecutedEvent ships on-chain, it isn't silently dropped the same way market_created_event and market_resolved_event were before this round of fixes.

Why

Closing out the same class of topic-naming drift across all four indexer parsers for consistency, now that #589 and #590 established the real convention from the contract source.

Test plan

• Updated apps/indexer/src/tradeParser.test.ts and apps/indexer/src/ingestion.test.ts's TRADE_TOPIC fixture to the corrected discriminator.
• npx vitest run apps/indexer/src — all pass except 3 pre-existing, unrelated failures already present on dev (collateralDepositedParser.test.ts, storage.test.ts).
• Updated docs/indexer-event-mapping.md and apps/indexer/fixtures/contract-event-vectors.json to document the corrected topic and note that this anticipates a not-yet-implemented on-chain event.

Closes #588