fix: update opinion base URL to proxy.opinion.trade:8443

[realfishsam]

Fixes #516

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Updates the Opinion exchange base URL from openapi.opinion.trade/openapi to proxy.opinion.trade:8443/openapi across all three layers: the vendor OpenAPI spec, the generated api.ts, and the config.ts default constant. Also updates the changelog to reflect the new URL.

Blast Radius

• core/specs/opinion/opinion-openapi.yaml -- vendor spec
• core/src/exchanges/opinion/api.ts -- generated API spec
• core/src/exchanges/opinion/config.ts -- default URL constant
• changelog.md -- user-facing documentation

Findings

1. Merge conflict with PR fix: add env fallbacks for hardcoded service URLs #589: PR 589 wraps the same DEFAULT_OPINION_API_URL in process.env.OPINION_API_URL || .... If PR 589 merges first, this PR must update the fallback URL inside the env var expression. If this PR merges first, PR 589 needs to wrap the new URL. Either way, manual conflict resolution is needed.

2. WebSocket URL not updated: The REST base URL moves from openapi.opinion.trade to proxy.opinion.trade:8443, but OPINION_WS_URL still points to wss://ws.opinion.trade. If the proxy also handles WebSocket traffic, this may need updating. If the WS endpoint remains on the original host, this is fine -- but it should be verified.

3. All three layers are correctly updated -- spec, api.ts, and config.ts are consistent with the new URL. The changelog documents the change. This is a clean URL migration.

PMXT Pipeline Check

• Field propagation: N/A
• OpenAPI sync: OK (api.ts matches spec)
• Type safety: N/A

Semver Impact

patch -- URL migration, no API shape change.

Risk

• WebSocket URL may also need updating (unverified).
• Merge conflict with PR 589.