feat: add covering indexes for capacity and cleanup queries

src/Trax.Effect.Data.Postgres/Migrations/032_capacity_cleanup_indexes.sql

@@ -0,0 +1,19 @@
+-- Indexes for LoadDispatchCapacityJunction and DeleteExpiredMetadataJunction query patterns.
+-- These reduce lock contention and improve query efficiency on shared database instances.
+
+-- Covering partial index for LoadDispatchCapacityJunction.
+-- The capacity query filters on train_state IN ('pending', 'in_progress'), excludes
+-- rows by name, and joins on manifest_id. INCLUDEing name allows the exclusion
+-- filter to be evaluated from the index without heap access.
+CREATE INDEX IF NOT EXISTS ix_metadata_active_capacity
+    ON trax.metadata (train_state, manifest_id)
+    INCLUDE (name)
+    WHERE train_state IN ('pending', 'in_progress');
+
+-- Partial index for DeleteExpiredMetadataJunction cleanup queries.
+-- The cleanup query filters WHERE name = ANY(@whitelist) AND start_time < @cutoff
+-- AND train_state IN ('completed', 'failed', 'cancelled'). This index covers
+-- terminal-state rows for efficient name + start_time filtering.
+CREATE INDEX IF NOT EXISTS ix_metadata_cleanup
+    ON trax.metadata (name, start_time)
+    WHERE train_state IN ('completed', 'failed', 'cancelled');