[codex] Add on-prem graph memory backend

[heikog]

What changed

• Adds a graph-memory adapter layer with Zep Cloud as the default backend.
• Adds a Graphiti bridge adapter for on-premise graph memory.
• Adds a separate Graphiti Bridge service backed by FalkorDB so Graphiti dependencies do not conflict with OASIS.
• Adds an optional docker compose profile for graphiti-falkordb and graphiti-bridge.
• Documents the on-premise graph-memory setup.

Why

Zep Cloud is expensive for this deployment. This keeps existing Zep behavior as the default while adding an on-premise path that can be enabled with GRAPH_MEMORY_BACKEND=graphiti_bridge.

Stack note

This PR is stacked on the i18n/prompt baseline branch so its diff stays focused on graph memory. After PR 666ghj#695 is merged, this branch can be rebased/retargeted to upstream main.

Validation

• python3 -m compileall -q backend/app graphiti_bridge
• docker compose --profile graphiti config
• git diff --check
• Manual smoke test on Paul using GraphitiBridgeGraphMemoryAdapter against graphiti-bridge/FalkorDB

[heikog]

Replaced by upstream PR 666ghj#696, which is based directly on upstream main.