feat: implement issues #485, #486, #487, #488

[emmanueltony792-blip]

#485: Add serializeBigInt helper for safe BigInt serialization in JSON

• Export serializeBigInt() as a named alias over sanitizeBigInts()
• Rewrite unit tests with Jest describe blocks covering all acceptance criteria

#486: Add structured debug log for price snapshot write

• Add ledgerSequence field to TradeEventPayload
• Emit debug log with creator_id, new_price, previous_price, ledger_sequence, written_at on every successful snapshot write (initial and update paths)
• Error log on failure was already present

#487: Enforce webhook max registration limit with 422

• Change service error from 409 to 422 for MAX_WEBHOOKS_REACHED
• Update existing integration test assertion to expect 422
• Add dedicated integration test file for the limit enforcement scenario

#488: Document ownership read model schema

• Add docs/indexer/ownership-read-model.md with table schema, update triggers, balance conservation invariant, and replay consistency behavior

Summary

Testing

• pnpm lint
• pnpm build
• pnpm exec prisma generate when schema or generated types changed

Checklist

• Linked issue or backlog item
• No secrets or live credentials added
• Docs updated if setup or env changed
• Change is scoped to one problem
• closes
• closes Add helper for safely serializing BigInt values in JSON API responses #485
• closes Add structured log for price snapshot write with creator ID and new price after each trade #486
• closes Add integration test for creator webhook max registration limit being enforced #487
• closes Add docs for ownership read model schema and how it is kept in sync with the indexer #488

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