feat(postgres): enforce distributed connection capacity

[Deathcharge]

Summary

• advance the internal PostgreSQL foundation to schema v4 with expiring per-socket connection leases
• atomically enforce deployment-wide and per-room capacity across replicas using database time and a transaction-scoped capacity lock
• bind renewal/release to the owning instance, reject archived rooms, reclaim expired sockets/owners, and prevent stable instance IDs from reviving phantom occupancy after a crash
• document the implemented boundary without exposing unfinished PostgreSQL mode through public configuration

Verification

• python -m ruff check .
• python -m ruff format --check .
• python -m mypy samsarix_chat_engine/postgres.py samsarix_chat_engine/postgres_connections.py samsarix_chat_engine/postgres_store.py
• python -m pytest -q -m "not postgres" → 139 passed
• python -m build
• python -m twine check dist/samsarix_chat_engine-0.12.0-py3-none-any.whl dist/samsarix_chat_engine-0.12.0.tar.gz
• live PostgreSQL concurrency/recovery tests run in the repository CI service job

Migration and rollback

Opening this internal foundation migrates compatible schemas to v4 by adding samsarix_connection_leases; existing room/message data is unchanged. The backend remains unselectable through public configuration. Rollback before public wiring requires stopping all candidate processes, dropping the v4-only connection-lease table, restoring schema metadata to v3, and deploying the prior exact commit.

Remaining gates

Presence transitions, distributed rate controls, public app wiring, real multi-process WebSocket tests, retained-log gap handling, load/soak, interruption, backup, and rollback drills remain required before any horizontal-scale claim.

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📥 Commits

Reviewing files that changed from the base of the PR and between f1ee158 and 8e7d9be.

📒 Files selected for processing (9)

• CHANGELOG.md
• ROADMAP.md
• docs/MULTI_INSTANCE_ARCHITECTURE.md
• samsarix_chat_engine/postgres.py
• samsarix_chat_engine/postgres_connections.py
• samsarix_chat_engine/postgres_store.py
• tests/conftest.py
• tests/test_postgres_connections.py
• tests/test_postgres_store.py

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