feat(postgres): coordinate ephemeral typing state

[Deathcharge]

Summary

• advance the internal PostgreSQL foundation to schema v6 with typing state bound by foreign key to admitted connection leases
• emit one durable coordination event on inactive→active, refresh deadlines without event storms, and make explicit stop owner-bound/idempotent
• sweep expired typing rows in bounded SKIP LOCKED batches and append stop events in the same transaction
• retain an internal origin connection ID for future sender exclusion while intentionally keeping typing events off the public relay until that contract is wired

Verification

• python -m ruff check .
• python -m ruff format --check .
• python -m mypy samsarix_chat_engine/postgres.py samsarix_chat_engine/postgres_typing.py
• python -m pytest -q -m "not postgres" → 139 passed
• 180 tests collected; live transition-only, owner-binding, bounded-expiry, event-order, and invalid-lease cases run in PostgreSQL CI

Migration and rollback

Opening the internal foundation adds samsarix_typing_states and advances compatible schema metadata to v6. Connection deletion cascades ephemeral rows; authoritative chat data is unchanged. Before public wiring, rollback requires stopping candidate processes, dropping the v6-only table, restoring metadata to v5, and deploying the prior exact commit.

Remaining gates

Lease-derived presence, sender-excluding public relay wiring, real multi-process sockets, reconnect/gap recovery, interruptions, load/soak, backup, and rollback drills remain required. Clients must continue honoring expires_in; a stop event is never the sole expiry mechanism.

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📥 Commits

Reviewing files that changed from the base of the PR and between b150c3e and de7e81e.

📒 Files selected for processing (8)

• CHANGELOG.md
• ROADMAP.md
• docs/MULTI_INSTANCE_ARCHITECTURE.md
• samsarix_chat_engine/postgres.py
• samsarix_chat_engine/postgres_typing.py
• tests/conftest.py
• tests/test_postgres_store.py
• tests/test_postgres_typing.py

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