Show start and end dates on the device/module/VM contract cards

[dr4gon123]

What

The contract cards on the device, module, and virtual machine detail pages currently show only the contract ID and SKU. This exposes the coverage window in the same card:

• Start Date — new start column on SupportContractAssignmentTable (contract__start)
• End Date — the table already had an end column (the assignment-level end override falling back to the contract's end, via the existing end_date property); the card was simply excluding it

So ContractMixin's include_columns becomes ['contract', 'sku', 'start', 'end'] and end leaves exclude_columns. renewal intentionally stays hidden — it's administrative rather than part of the coverage window, and the card should stay compact.

Why

The coverage window is the first thing anyone asks about a support contract. Today it requires clicking through to each contract from the card, even though the data (and the table columns for it) are already right there — the card just filters them out.

Notes

• No migrations; two files plus the table registration, ~8 lines.
• Tabbed active/expired/future/unspecified behavior is unchanged.
• Happy to adjust column order/labels if you'd prefer a different presentation (e.g. a single combined start – end column).