DRU-856 -- Refuse a create at a provider's host limit

[czpython]

HOST_LIMITS sets the most live hosts per provider, as JSON (for example {"exe": 28, "docker": 6}). A provider that it does not name has no limit.

Behavior

• create_host takes pg_advisory_xact_lock keyed by the provider name, in the transaction that inserts the row. The lock serializes the creates of one provider, so two requests cannot both pass the count.
• The count includes the provider's hosts that are not in error and have not expired. Provisioning rows count. Warm pool hosts count, because the pool maintainer creates through the same path.
• At the limit, POST /hosts returns 503 with CAPACITY and Retry-After: 30. The refused create leaves no row.
• SQLite has no advisory locks, so it runs the count only.

Checks

• One service test: exe at its limit refuses, stub with no limit creates.
• uv run pytest passes on SQLite and on Postgres 17 (785 tests).
• ruff check, ruff format --check, and pyright pass.