fix(engine): enrich only newly created opportunity sessions

[xeronsh]

What

Duplicate pulse events (EventID repeats) no longer spawn a second async enrichment goroutine on the same OpportunitySession.

HandlePulseEvent returns (nil, nil) when the session already exists, but the aggregator treated a nil error as "enrich again". Now enrichment only starts when a session was actually created:

sess, err := p.opportunity.HandlePulseEvent(evt)
if err != nil {
    ...
} else if sess != nil {
    p.enrichOpportunityAsync(evt)
}

Why

Two watch goroutines on one session means double OHLCV fetching, duplicate cycle work, and two writers racing to attach tickets to the same session. Non-blocker caught in PR #4 review; this is the cleanup PR.

Verification

• TestEventAggregator_DuplicatePulseEnrichesOnce: feeds the same market_impulse EventID twice, asserts exactly one enrichment spawn (fails with 2 spawns on the old code — confirmed)
• make check green (build / vet / fmt / mod / lint / test -race)

Closes the follow-up from #4 review.