Design and migrate time-series schema for daily network metrics

[christabel888]

Context: Migrations in this repo are numbered sequentially (001_... through 030_comprehensive_seed_data.sql); this issue is the actual schema work several endpoint issues above depend on (#15-#19).

Where:

• New migrations/031_create_network_metrics_timeseries.sql (+ down migration if following the pattern of migrations 001-002)

What to do:

• Table(s) for daily DAA, tx-count, volume, new-accounts, fee-trends -- one row per metric per day, or one wide table, whichever matches how Add /api/v1/network/daily-active-accounts time-series endpoint #15-Add /api/v1/network/fee-trends endpoint #19 query it.

Suggested approach:

1. Design one wide table or several narrow per-metric tables for DAA/tx-count/volume/new-accounts/fee-trends -- check what shape Add /api/v1/network/daily-active-accounts time-series endpoint #15-Add /api/v1/network/fee-trends endpoint #19's queries will actually want before committing (a wide table is simpler to query for "give me everything for date X"; narrow tables are simpler to extend later).
2. Add appropriate indexes (date column at minimum) up front, verified via Add index verification for new time-series tables in scripts/verify_indexes.sh #50.

Watch out for:

• Get the schema right the first time if you can -- Add /api/v1/network/daily-active-accounts time-series endpoint #15-Add /api/v1/network/fee-trends endpoint #19 will be built directly against this, and a later schema change means touching all of them again.

Definition of done:

• Migration applies cleanly on top of migration 030
• Add /api/v1/network/daily-active-accounts time-series endpoint #15-Add /api/v1/network/fee-trends endpoint #19 can query it directly