perf: replace O(n²) concat with push in metaculus fetchEvents

[realfishsam]

Fixes #552

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Replaces all = all.concat(results) with all.push(...results) in the metaculus fetchEvents.ts pagination loop, mirroring the same O(n^2) fix applied in PR #571.

Blast Radius

• Core: core/src/exchanges/metaculus/fetchEvents.ts only
• No OpenAPI, SDK, or type changes

Findings

1. Same let -> could-be-const note as PR perf: replace O(n²) concat with push in metaculus fetchMarkets #571 (line 31). Non-blocking.
2. Batch size is 100, safely below V8 spread argument limits.
3. Both fetchMarkets and fetchEvents share the same fetchPostPages pattern. If these were ever consolidated into a shared helper, only one fix would be needed. Minor tech-debt observation, not blocking.

PMXT Pipeline Check

• Field propagation: N/A
• OpenAPI sync: N/A
• Type safety: OK

Semver Impact

patch -- internal perf improvement, no API or behavioral change

Risk

Not verified against live Metaculus data. Mechanically identical to PR #571.