Background Task Queue Service

Problem/Feature Description

A small team needs a lightweight background task queue implemented in Go with SQLite as the backing store. This replaces Redis/RabbitMQ for a low-traffic internal tool. The service runs as an HTTP server that accepts tasks and processes them in the background.

The service needs to support:

• Submitting a new task via HTTP POST with a task type, JSON payload, and optional priority (1-5, default 3)
• Claiming the next available task for processing (atomically marks it as "processing" so no other worker picks it up)
• Marking a task as completed or failed (failed tasks include an error message)
• Retrying failed tasks (up to 3 attempts, then marked as "dead")
• Listing tasks by status (pending, processing, completed, failed, dead)
• Getting task details by ID

Each task has: id, task_type, payload (JSON text), priority, status, attempts, max_attempts, error_message, created_at, updated_at, completed_at.

The claim operation is critical -- it must atomically find the highest-priority pending task and mark it as processing in a single transaction to prevent race conditions.

Output Specification

Produce:

• main.go -- HTTP server with routes
• db.go -- Database connection and migrations
• queue.go -- Task queue operations (submit, claim, complete, fail, retry, list)
• handlers.go -- HTTP handlers
• go.mod -- Module file

The code should be complete Go with no placeholders or TODO comments.