Add index on grants table for grantee_address query performance

[Copilot]

The GET /grantees/:address/users endpoint was doing a full table scan on grants — the table only had a primary key on (user_id, txhash, grantee_address) with no usable index for lookups by grantee_address.

Changes

• ddl/migrations/0191_grants_grantee_address_idx.sql — adds a partial index covering the query's filter and sort columns:

  CREATE INDEX IF NOT EXISTS idx_grants_grantee_address
  ON grants(grantee_address, is_revoked, created_at DESC)
  WHERE is_current = true;

  The WHERE is_current = true predicate keeps the index small; is_revoked covers the boolean filter; created_at DESC supports ORDER BY … LIMIT … OFFSET pagination without a sort step.

• sql/01_schema.sql — schema snapshot updated to reflect the new index.

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