fix: add env fallbacks for hardcoded service URLs

[realfishsam]

Fixes #508

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Adds environment variable fallbacks for three hardcoded service URLs: Opinion API/WS URLs in config.ts, the PMXT Router API URL in router/client.ts, and the Polymarket Goldsky subgraph URL. Also extracts two magic numbers (maker/taker trade limits) into named constants.

Blast Radius

• core/src/exchanges/opinion/config.ts -- Opinion exchange config
• core/src/router/client.ts -- Router API client (all venues)
• core/src/subscriber/external/goldsky.ts -- Polymarket subscriber

Findings

1. Merge conflict with PR fix: update opinion base URL to proxy.opinion.trade:8443 #591: Both PRs modify core/src/exchanges/opinion/config.ts line 1. PR 589 adds process.env.OPINION_API_URL || "https://openapi.opinion.trade/openapi" while PR 591 changes the hardcoded URL to "https://proxy.opinion.trade:8443/openapi". If both merge, the env var name from 589 and the default URL from 591 must both survive. Whichever merges second will need a conflict resolution.

2. Env var name mismatch with changelog: The changelog (updated in PR 591) documents OPINION_BASE_URL as the env var for Opinion. PR 589 introduces OPINION_API_URL and OPINION_WS_URL instead. Consumers reading the changelog would set the wrong var. The naming should be reconciled.

3. Router env var PMXT_API_URL is reasonable -- it only sets the default; the constructor still accepts an explicit baseUrl parameter that takes precedence via the ?? operator. No behavioral change when the env var is unset.

4. Goldsky constant extraction is clean -- pure refactor replacing magic numbers 5 and 20 with named constants, no behavioral change.

PMXT Pipeline Check

• Field propagation: N/A (no new fields)
• OpenAPI sync: N/A
• Type safety: OK

Semver Impact

patch -- env var fallbacks are additive; existing behavior unchanged when vars are unset.

Risk

• Merge ordering with PR 591 needs manual attention.
• OPINION_API_URL vs OPINION_BASE_URL naming inconsistency could confuse users.