Leaderboards: personal-best, time window, Friends scope

[trriplejay]

Part of #17. Blocked by: #22, #19 (needs Race results to rank, and Friends for the Friends-scoped view).

Scope

Queries + handler for a Race Type's Leaderboard:

• Ranked by each racer's own personal-best result only (not every attempt).
• Filtered by time window: All-Time / Month / Week — each its own ranking, not combined.
• Filtered by scope: Global, or Friends-only (a filter over the same underlying data, not a separate structure).
• DNF results excluded.

Geography-based ("in your area") scope is explicitly out of scope — deferred, undesigned.

Includes adding the db/queries/*.sql this service needs and running make sqlc-generate — the schema issue (#18) only creates migrations, it doesn't pre-write queries. Follow the existing db/queries/users.sql pattern: queries scoped to what this service actually asks the DB, not generic per-table CRUD.

Testing

• Integration tests (-tags integration) covering personal-best selection, each time window boundary, and Friends-scope filtering.
• Handler: unit tests with narrow interfaces + httptest.

See CONTEXT.md (Leaderboard).