Add leader election recovery with generation isolation

[ssteele110]

Summary

• Add leader election architecture to handle master worker death during queue population
• Add MasterDied exception for detecting master failures during setup
• Add configurable master_lock_ttl (30s default) and max_election_attempts (3 default)
• Use SET NX EX for master lock with TTL instead of SETNX (which had no TTL)
• Namespace all queue data keys by generation UUID for complete isolation
• Add retry loop with automatic re-election when master dies

How It Works

1. Master acquires lock with SET key "setup:{uuid}" NX EX 30 (30s TTL)
2. If master dies mid-population, lock expires automatically
3. Workers detect lock expiry → MasterDied exception
4. Any worker can become new master with new generation UUID
5. New population uses isolated namespace: build:{id}:gen:{uuid}:*
6. Old workers detect generation staleness and exit gracefully

Test plan

• Verify existing tests pass
• Test master death scenario: start master, kill mid-population, verify new master elected
• Verify workers on old generation detect staleness and exit
• Test 2 sequential master deaths within 120s timeout
• Verify max_election_attempts limit works (fails after 3 attempts)

🤖 Generated with Claude Code