Fix/backend security remittance score rate limit

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  Closes [Backend] scoreDecayService reads/writes a non-existent borrowers table, so inactivity decay always throws #1095
  Closes [Backend] createRemittance builds the payment XDR with a hardcoded source sequence of 0, so the signed transaction is rejected with txBAD_SEQ #1096
  Closes [Backend] Score-update rate limiter uses a non-atomic GET-then-SET, letting concurrent requests exceed the per-user cap #1097
  Closes [Security] Indexer webhook and recent-events endpoints accept any scoped API key, bypassing admin:webhooks/admin:indexer scope enforcement #1098