Initializes DynamoDB persistence layer into sync flow

[tamaradement]

PR Description

• Adds a new DYNAMODB_TABLE_NAME environment variable.
• Begins integrating the DynamoDB persistence layer into the main sync flow.
  • For each filtered Clockify request, checks whether the request already exists in DynamoDB and logs the result.

Steps I Took to Verify

• Ran the sync script against a newly created Clockify request and verified the request was reported as not existing in DynamoDB.
• Inserted a corresponding item into DynamoDB and ran the same script again. Verified the request was reported as already existing in DynamoDB.

[tamaradement]

My thinking here is that this is just the first step in integrating the persistence layer. The next change will use this to filter out requests that have already been processed before creating gCal events. Once that's in place, I'll probably move this logic into core so main.go stays orchestration-focused.

[dacut]

LGTM!