db: add partial indexes on players for active=true query path

Drop redundant plain-ASC rating indexes (players_standard/rapid/blitz_idx)
since Postgres can scan the existing DESC NULLS LAST indexes backwards for
ASC queries.

Add WHERE active=true partial indexes for all API-facing queries:
- players_standard_active_idx
- players_rapid_active_idx
- players_blitz_active_idx
- players_federation_id_active_idx

The planner will prefer these smaller, denser indexes over the full
players_*_desc_idx variants whenever the query includes active=true,
which is the case for all public API endpoints.

Author: lenguyenthanh

modules/db/src/main/resources/db/migration/V0020__active_partial_indexes.sql

@@ -0,0 +1,16 @@
+-- Drop plain ASC rating indexes: redundant because Postgres can scan DESC indexes
+-- backward for ASC queries, and partial indexes below cover the hot path.
+DROP INDEX IF EXISTS players_standard_idx;
+DROP INDEX IF EXISTS players_rapid_idx;
+DROP INDEX IF EXISTS players_blitz_idx;
+
+-- Partial indexes scoped to active players.
+-- All API queries filter WHERE active = true; the planner will prefer these
+-- smaller, denser indexes over the full players_*_desc_idx alternatives.
+CREATE INDEX players_standard_active_idx ON players (standard DESC NULLS LAST) WHERE active = true;
+CREATE INDEX players_rapid_active_idx    ON players (rapid    DESC NULLS LAST) WHERE active = true;
+CREATE INDEX players_blitz_active_idx    ON players (blitz    DESC NULLS LAST) WHERE active = true;
+
+-- Partial index for federation-scoped lookups (replaces players_federation_id_idx for
+-- the active-player query path; the full index is kept for crawler/admin queries).
+CREATE INDEX players_federation_id_active_idx ON players (federation_id) WHERE active = true;