Add Redis subscribe usage ingest

[Willxup]

Summary

• Add Redis SUBSCRIBE usage ingest with Redis/HTTP fallback into the existing redis_usage_inboxes buffer.
• Split remote ingest from local inbox processing so usage decoding and aggregation stay unchanged.
• Harden Redis RESP parsing limits and ignore local AGENTS.md instructions.

Test plan

• go test ./internal/poller/... ./internal/cpa ./internal/app
• go test ./...
• Restarted backend locally with project .env and verified ping endpoint during testing

[gemini-code-assist]

Code Review

This pull request refactors the usage ingestion architecture by replacing the legacy RedisDrain with a more modular system comprising a RedisIngestRunner and a RedisProcessRunner. The new implementation introduces a state machine that supports Redis SUBSCRIBE, Redis pull, and HTTP pull with built-in fallback and recovery logic. Additionally, the Redis client now includes security limits for RESP parsing to improve stability. Review feedback highlighted several areas for improvement: ensuring the backfill logic fully drains queues during startup to prevent data lag, allowing the ingest runner to "upgrade" back to higher-priority modes if they become available after a fallback, and making the Redis subscription channel configurable to ensure proper data isolation in shared environments.