perf(db): index session and challenge expiry columns

The sliding-30-day session reaper and the passkey-challenge sweep both
filter on expires_at, but neither sessions nor auth_challenges indexed
that column, so the cleanup degraded to a sequential scan on a busy
instance. Every sibling expiry table already indexes its expiry column;
add the two missing indexes via migration 0178.

prisma/migrations/0178_v1187_session_expiry_indexes/migration.sql

@@ -0,0 +1,19 @@
+-- v1.18.7 — expiry indexes for the two tables swept by the session reaper.
+--
+-- `sessions` (sliding-30-day session expiry) and `auth_challenges` (passkey
+-- registration / authentication challenges) are both pruned on `expires_at`,
+-- but neither carried an index on that column. On a busy multi-user instance
+-- the cleanup degraded to a sequential scan. Every sibling expiry table
+-- (`refresh_tokens`, `whoop_oauth_states`, `idempotency_keys`,
+-- `withings_oauth_states`, …) already indexes its expiry column; this brings
+-- the two stragglers in line. Additive; no data touched.
+--
+-- Reversibility (down):
+--   DROP INDEX IF EXISTS "sessions_expires_at_idx";
+--   DROP INDEX IF EXISTS "auth_challenges_expires_at_idx";
+
+-- CreateIndex
+CREATE INDEX IF NOT EXISTS "sessions_expires_at_idx" ON "sessions"("expires_at");
+
+-- CreateIndex
+CREATE INDEX IF NOT EXISTS "auth_challenges_expires_at_idx" ON "auth_challenges"("expires_at");

prisma/schema.prisma

@@ -721,6 +721,9 @@ model Session {
   user User @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@index([userId])
+  // v1.18.7 — the sliding-30-day session reaper sweeps on `expiresAt`; without
+  // this index the cleanup degrades to a sequential scan on a busy instance.
+  @@index([expiresAt])
   @@map("sessions")
 }
 
@@ -735,6 +738,9 @@ model AuthChallenge {
   user User? @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@index([userId])
+  // v1.18.7 — challenges are swept by expiry; index the column the reaper
+  // filters on so the cleanup stays index-backed.
+  @@index([expiresAt])
   @@map("auth_challenges")
 }