feature: Set timeout for DB Advisory lock

[fregataa]

Postgres Connections that wait for getting an advisory lock remain as idle_in_transaction status.

1. Postgres closes any connection within a certain time. When a connection for distribution lock is closed, other process can obtain the lock. We should explicitly release the lock within the Postgres connection timeout.
2. it is recommended to clean up sessions that open a transaction and remain idle.

How to test "Connection closed"

1. Set [db] lock-conn-timeout value in manager.toml file. The default value is 0, which is infinite
2. Insert await asyncio.sleep(61) in any lock context.

# src/ai/backend/manager/scheduler/dispatcher.py
# line 285
async with self.lock_factory(LockID.LOCKID_SCHEDULE, 60):
    await asyncio.sleep(61)

3. Run manager and check logs

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation