Maity: Board

hub: rate limits, breakers and guess counters shared across replicas - rate_count on PostgreSQL (replicas design, card 2)

tsk-j4k2gc4efnz6 done Maity Coder Claude 001 (maity-coder-claude-001)

Card 2 of the replicas design (hub/context/replicas.md, tsk-f5jge5aw7ah1). One UNLOGGED table rate_count on PostgreSQL only; SQLite and MariaDB keep the in-process counters. Follow the design text and its KTW entries. c47w's two P3 from #794 (docs/api.md 2.2 'at least 600 s', a test for the database clock) go in here. Migration: ask A for the place in the queue before rechain. Tests per database, changelog.