fix(gateway): security hardening — auth guards, ownership checks, validation, rate limiting

docs/reports/code-review/gateway-security-20260313.md

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+# Code Review Report — Gateway Security Hardening
+
+## Scope Reviewed
+
+- `apps/gateway/src/chat/chat.gateway-auth.ts`
+- `apps/gateway/src/chat/chat.gateway.ts`
+- `apps/gateway/src/conversations/conversations.dto.ts`
+- `apps/gateway/src/chat/__tests__/chat-security.test.ts`
+
+## Findings
+
+- No blocker findings in the final changed surface.
+
+## Review Summary
+
+- Correctness: socket auth helper now returns Better Auth session data unchanged, and gateway disconnects clients whose handshake does not narrow to a valid session payload
+- Security: conversation role validation now rejects `system`; conversation content ceiling is 32k; chat request ceiling remains 10k
+- Testing: targeted auth, ownership, and DTO regression tests pass
+- Quality: `pnpm typecheck`, `pnpm lint`, and `pnpm format:check` all pass after the final edits
+
+## Residual Risk
+
+- `chat.gateway.ts` uses local narrowing around an `unknown` session result because the requested helper contract intentionally returns `unknown`.