Add historical top-movers snapshot storage for trend/share cards

[christabel888]

Context: For share cards / trend sparklines on Top Movers, a single current ranking isn't enough -- need history.

Where:

• New migration, e.g. migrations/032_create_top_movers_snapshots.sql, following the existing NNN_description.sql naming convention (latest is 030_comprehensive_seed_data.sql)

What to do:

• Store each hourly run of Add hourly background job to compute top movers in src/jobs #34's job as a row, so a 7-day trend can be reconstructed later.

Suggested approach:

1. New migration, e.g. migrations/032_create_top_movers_snapshots.sql, following the NNN_description.sql convention (latest is 030_comprehensive_seed_data.sql).
2. One row per (asset, hour) from Add /api/v1/rankings/top-movers endpoint: 24h percent change by holders/volume #33/Add hourly background job to compute top movers in src/jobs #34's hourly job -- this becomes the data source for share-card sparklines later.

Watch out for:

• Decide retention for this table up front (it grows one row per asset per hour, indefinitely) -- coordinate with Add data-retention policy for new time-series tables, aligned with BACKUP_RETENTION_DAYS #48's retention policy issue rather than leaving it unbounded.

Definition of done:

• Migration adds the snapshot table with appropriate indexes
• Job from Add hourly background job to compute top movers in src/jobs #34 writes to it