Add group-by endpoint for vault analytics

[Jayy4rl]

Context: Analytics dashboards need vault metrics grouped by category or state without client-side aggregation.

Requirements:

• GET /api/v1/analytics/vaults/group-by?by=rwa_category returns [{ group; vaultCount; totalValueLocked; averageApy }].
• Support by values: rwa_category, state, maturityMonth. Return HTTP 400 for invalid values.

Acceptance criteria:

• Grouping by state returns one row per non-empty state.
• totalValueLocked is the sum of total_assets within each group.