feat(postgres): enforce distributed rate buckets

[Deathcharge]

Summary

• advance the internal PostgreSQL foundation to schema v5 with message, search, and typing rate buckets
• atomically enforce a fixed database-time allowance per scope/identity across replicas while keeping hot identities on row-level contention
• bound new-key cardinality under a dedicated advisory lock, prune expired buckets, and fail closed at capacity
• persist only scope-separated SHA-256 key digests and explicitly document that hashing is data minimization rather than anonymization

Verification

• python -m ruff check .
• python -m ruff format --check .
• python -m mypy samsarix_chat_engine/postgres.py samsarix_chat_engine/postgres_rate_limits.py
• python -m pytest -q -m "not postgres" → 139 passed
• live PostgreSQL concurrent exact-limit, independent-scope, privacy-shape, capacity, and expiry tests run in CI

Migration and rollback

Opening the internal foundation adds samsarix_rate_buckets and advances compatible schema metadata to v5; authoritative room/message data is unchanged. The backend remains unavailable through public configuration. Pre-wiring rollback requires stopping candidate processes, dropping the v5-only table, restoring metadata to v4, and deploying the prior exact commit.

Remaining gates

Presence/typing expiry, public request-path wiring, real multi-process tests, retained-log gaps, interruption, load/soak, reconnect storms, backup, and rollback drills remain required before any horizontal-scale claim. Fixed-window boundary behavior is deliberate and documented; load evidence must validate it.

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📥 Commits

Reviewing files that changed from the base of the PR and between c688005 and 8222e5a.

📒 Files selected for processing (8)

• CHANGELOG.md
• ROADMAP.md
• docs/MULTI_INSTANCE_ARCHITECTURE.md
• samsarix_chat_engine/postgres.py
• samsarix_chat_engine/postgres_rate_limits.py
• tests/conftest.py
• tests/test_postgres_rate_limits.py
• tests/test_postgres_store.py

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