Fix Photon conversation persistence

[gitcommit90]

Summary

• keep one durable active Photon conversation per mapped sender and channel
• append every ordinary inbound text to the same 1Helm thread across connector restarts and Photon space-ID changes
• make exact /new the sole reset boundary without invoking the resident
• migrate the latest existing Photon sender thread forward on upgrade
• retain reply deduplication and handle /new before any conversation exists

Verification

• npm run typecheck
• npm run build
• npm test
• git diff --check

All verification ran from a fresh archive of the exact staged Git tree.