Add import contracts command

[mayasimi]

resolves #475

Changes

• Adds import_contracts with --file / --input support.
• Reads contracts from the export_contracts JSON format.
• Supports plain address/name mappings, including { "CONTRACT_ID": "Name" }.
• Creates new TrackedContract records.
• Skips existing contracts without erroring or overwriting them.
• Supports --owner for assigning imported contracts to an existing user.
• Falls back to a deterministic soroscan-import service user when no owner is provided.
• Adds tests for import success, duplicate handling, owner selection, mapping formats, and validation errors.

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