fix: route Composio user identity for Gmail hydration

[irresi]

Summary

• persist Composio's signed webhook user_id alongside the connected-account route
• pass both userId and connectedAccountId to Gmail tool execution, matching the live Composio API contract
• upgrade empty legacy inboxes and fail closed when queued legacy rows cannot be safely hydrated
• document bounded live-canary evidence and qualified GBrain compatibility

Why

The first read-only live canary returned Composio error 1811: tool execution requires the user/entity identity together with the connected account. The original synthetic client accepted connected account alone and therefore missed this production contract.

Verification

• bun run check — Biome, TypeScript, 47 tests, production build
• bun audit — no vulnerabilities
• gitleaks git --staged --redact --no-banner — no leaks
• operator-approved live canary: query 3 IDs, hydrate 1, metadata-only write 1, GBrain read-back and search both verified
• live canary used GBrain 0.42.67; 0.42.56 is known incompatible with the source-scoped post-write verification path

Deployment state

No long-running worker, trigger, public webhook, or Gmail body retention was enabled. The canary-created career-email source contains one metadata-only Gmail page.