perf(viewer): hydrate session posts

[benvinegar]

Summary

• Add ?hydrate=1 support to the session posts endpoint so the viewer can load full post details in one request.
• Update session open and live reconnect resync to use the hydrated endpoint, with a fallback to legacy per-post fetches if needed.
• Add API and Chromium e2e coverage that verifies session open avoids N+1 GET /api/posts/:id calls.

Review

• Ran a fresh-context subagent code review; no blockers were found.

Tests

• npm run typecheck
• npm run lint
• npm run format:check
• npm test
• npx playwright test e2e/viewer.spec.ts --project=chromium

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