Refactor ban management behind BanMgr interface

Extract ban logic into a BanMgr interface with two implementations:
- BanFile: file-based YAML storage with support for IP, username, and
  nickname bans (backwards-compatible with legacy format)
- RedisBanMgr: Redis-backed implementation with permanent and temporary
  ban support, using fail-safe deny-on-error behavior

This replaces scattered Redis calls in API handlers, transaction
handlers, and server connection logic with unified interface calls,
removing the Redis dependency from the API server constructor and
enabling ban functionality for both file-only and Redis deployments.

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