Feat/removing postgresql

[quentinP-hexamind]

Drop Postgres as the default & delegate conversation history to the backend

Summary

Two related simplifications to how the platform stores state:

1. SQLite is now the default datastore for the proxy, that means no server to stand up. Postgres becomes an opt-in extra for multi-worker / high-concurrency deployments.
2. The agent backend now owns conversation memory. The proxy forwards only the new user turn and keeps its stored transcript purely for display (sidebar / reload). A new /forget endpoint lets the backend drop a conversation's memory when the user deletes it.

What changed

Datastore (proxy)

• Default [PLATFORM_DATABASE_URL] → sqlite+aiosqlite:///./hexa.sqlite (config.py).
• aiosqlite promoted to a core dependency; asyncpg moved behind a new postgres extra (pip install '.[postgres]') (pyproject.toml).
• docker-compose.yml, .env.example, and QUICKSTART.md reframed: Postgres is now optional, with the opt-in steps documented. *.sqlite git-ignored.

Conversation memory (proxy)

• The chat route forwards only [{role: user, content: …}], the prior _assemble_history DB read is gone ([chat.py]). Stored user/assistant rows are now explicitly the display transcript, never model context.
• Deleting a conversation calls the backend's /forget (best-effort, a down/unimplemented backend never fails the delete) ([conversations.py], [runtime_client.py]).

Reference & template backends

• New in-process memory store + /forget route in the reference agent-server ([memory.py], [routes/agents.py]), with _assistant_text capturing the reply across all framework event shapes.
• Same pattern added to the copy-me starter-agent and hexgate-agent templates.
• verify_backend.py now probes /forget.

Contract

• CONTRACT.md §5 documents the new ownership model, the conversation_id memory key, cold-id handling, and the /forget lifecycle.

Behavior contractual change

Any custom backend must now:

• treat input.messages as the new turn only and own its own memory keyed by context.conversation_id (accepting unseen ids without error);
  implement POST /agents/{id}/forget {conversation_id} → {forgotten: bool}

Testing

• proxy-server: 92 passing, test_chat.py updated to assert only the new turn is forwarded while the full display transcript is still stored; test_conversations.py adds delete→/forget coverage incl. the best-effort failure path.
• agent-server: 25 passing (new suite), memory.py, the stream route's transcript reconstruction, /forget, and _assistant_text per framework. Wired into make test (so it runs in CI).

Follow-ups (not in this PR)

• starter-agent / hexgate-agent memory logic is currently untested.
• No retry-idempotency dedup by run_id (documented simplification); a missing conversation_id currently yields empty history rather than falling back to input.messages