Add stats command for executions and datasets

[lewisjared]

Summary

Add a stats subcommand to both the executions and datasets CLI groups to provide a quick operational overview of the current state of the database.

Motivation

Currently, understanding the state of executions or datasets requires eyeballing list-groups or list output. A summary command would give operators a quick dashboard view, especially useful when monitoring long-running evaluation campaigns.

Proposed Commands

ref executions stats

Show counts of executions grouped by status and provider/diagnostic:

Provider     Running  Failed  Successful  Not Started  Dirty
pmp               2       3          45            0      1
esmvaltool        0       1          32            5      5
ilamb             1       0          18            2      0

Should support the same --diagnostic and --provider filters as list-groups.

ref datasets stats

Show counts of datasets grouped by dataset type, with breakdowns by key facets (e.g., source_id, experiment_id):

Dataset Type    Count  Finalised  Unfinalised
cmip6             234        180           54
obs4mips           12         12            0

Acceptance Criteria

• ref executions stats shows per-provider counts of running/failed/successful/not-started/dirty
• ref datasets stats shows per-dataset-type counts with finalisation status
• Both commands support relevant filters (--provider, --diagnostic, etc.)
• Tests for both commands