fix(indexer): use indexed exact-match participant columns instead of ILIKE on JSON

[sasasamaes]

Closes #889

Summary

Participant queries in backend/src/services/eventIndexer.js (queryEventsByPublicKey, queryEventsByType, getEventStats) previously used payload::text ILIKE '%<publicKey>%' — a full-text substring scan over the entire contract_events table. This caused:

1. Performance: full-table scan on every dashboard call, unusable by any index.
2. Correctness: a public key (55 chars) appearing in unrelated payload fields (e.g. a memo or nested array) produced false matches — users could see events they weren't part of.

The table already stores parsed from_addr / to_addr columns (populated on insert via eventParser.js), but the query helpers never used them.

Changes

• Rewrite all three query helpers to match exactly with from_addr = $1 OR to_addr = $2
• Add migration 028_events_participants_index.js: cross-dialect composite index idx_events_participants (from_addr, to_addr) (uses CREATE INDEX IF NOT EXISTS, has down() rollback per CONTRIBUTING.md)
• Keep the in-memory fallback store in sync — now uses exact column match too
• Add _seedForTest helper to seed events into the in-memory store for tests

Tests

New suite backend/__tests__/eventIndexer-column-queries.test.js (13 tests):

• Exact-participant filtering returns only the user's events (queryEventsByPublicKey, queryEventsByType)
• A public key string appearing in a memo does NOT match (false-positive regression test)
• getEventStats counts only true participants and does not inflate from memo matches
• Descending ledger order, unknown-key empty results

All 13 new tests pass; existing 15 integration-eventIndexer tests still pass; eslint src/**/*.js reports no issues in changed files.

Acceptance Criteria

Criteria	Status
Participant queries use indexed exact-match columns	✅
Memo containing a public key no longer produces false matches	✅
≥4 tests; lint clean	✅ (13 tests)
In-memory fallback kept in sync, documented as best-effort	✅

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