[codex] Skip raw session recall when session save is disabled

[Bosn]

Summary

• extend MNEMO_DISABLE_SESSION_SAVE so raw session rows do not participate in /memories recall/list paths
• skip session fan-out in default confidence recall and return empty results for explicit memory_type=session when the server switch is enabled
• map client-canceled request contexts to HTTP 499 instead of logging them as internal 500s

Why

Local logs showed GET /memories requests reaching client cancellation around 30s while session recall was still running despite MNEMO_DISABLE_SESSION_SAVE=true. This keeps the switch aligned with the intended facts-only mode and makes canceled requests easier to distinguish from server failures.

Validation

• cd server && go test -race -count=1 ./internal/config/
• cd server && go test -race -count=1 ./internal/handler/
• cd server && go test -race -count=1 ./internal/service/