fix(v2.3.1): typed session status, HTTP 500 on DB errors, README sync

[ek33450505]

Summary

• Type fix: sessions.ts SessionWithStatus type extension replaces as unknown as Record<string, unknown> double-cast
• Error responses: completenessEvents, codeRefChecks, fileWrites routes now return HTTP 500 on caught exceptions (consistent with costSummary.ts; previously returned HTTP 200 with empty payload)
• README: Version badge updated to v2.3.1; pages table expanded to 11 pages; agent count updated to 33; new API routes documented; DB tables section updated; System tab description updated
• Version: Bumped package.json to v2.3.1

Test plan

• npx tsc --noEmit passes clean
• GET /api/sessions — response includes status field on sessions with DB fallback data
• GET /api/completeness-events?limit=abc — returns HTTP 500 (not HTTP 200 empty)
• README renders correctly on GitHub — version badge shows 2.3.1, all 11 pages listed

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