Add date-ranged usage query method

[nsheff]

What this does

Adds a new way to read BEDbase usage counts (BED/BEDset views, searches, file downloads) for a chosen date range, without the existing 20-item cap.

Today the only usage readout (get_detailed_usage) returns just the top 20 items summed over all time. The stored data is actually per-item and time-stamped, but there was no way to query it by date or to get more than 20 rows.

Changes

• New BedBaseAgent.get_usage(event_type, date_from, date_to, limit, offset) that groups usage by item, sums the counts, optionally filters to a date window, and pages results. Read-only.
• Two small response models (UsageItem, UsageResponse).

The existing get_detailed_usage is left unchanged.

Why

Enables accurate per-month usage reporting (used by the bedhost /v1/usage endpoint in a companion PR) instead of a top-20 all-time snapshot.

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Pull request overview

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Suppressed comments (2)

bbconf/bbagent.py:494

• UsageResponse.count is currently set to len(results), which becomes the page size after limit/offset rather than the total number of matching keys. Also, NULL/empty keys are filtered out in Python (if f[0]), which means pagination can be inconsistent (limit/offset are applied before filtering) and clients may receive fewer than limit items unexpectedly.

            count=len(results),

bbconf/bbagent.py:450

• The docstring states date_from <= date_to, and limit/offset are used for pagination, but there is no input validation. Passing date_from > date_to or negative limit/offset will currently produce confusing results (empty pages or DB-dependent behavior) rather than a clear error.

        if event_type not in usage_map:
            raise ValueError(
                f"Invalid event_type '{event_type}'. Must be one of: "
                f"{', '.join(usage_map)}."
            )