Implement `run_due_checks` with Lambda Logging

[JohnBrennan95]

Implementation Details

• Use the ping task from Mu; can initially reside in app.py.
• Attach a thread from Lambda’s outer shell to the function so it can be referenced later.
• Query all due monitors at next_due_at.
• Implement a secondary partition key and a secondary sort key (next_due_at) for efficient queries.
• Each monitor should trigger run_check via concurrent Lambda calls.
• All functionality must be tested with Moto; no direct AWS deployment yet.
• Refer to architecture.md for implementation guidance.

[calebsyring]

Closed via #49