Refactor main.ts to extract specific functionalities

[dtinth]

Refactors src/main.ts by extracting and organizing code into separate, more focused modules.

• Extracts environment variables: Moves environment variable definitions to src/env.ts.
• Schemas: Moves Task and TaskListFile JSON schemas to src/schema.ts.
• Context creation: Extracts the creation of the context object, which includes DynamoDB and SQS clients, to src/createContext.ts.
• Duration tracking: Separates the duration tracking functionality into src/createDurationTracker.ts.
• DynamoDB functions: Moves DynamoDB related functions, such as table creation and task status updates, to src/db.ts.
• SQS functions: Extracts SQS related functions, including queue creation and message visibility timeout updates, to src/queue.ts.
• Main module cleanup: Updates src/main.ts to import and use the newly created modules, significantly reducing its complexity and improving readability.

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