feat: add separate read/write allowlists for granular access control

- Split allowed_npubs into allowed_npubs_read and allowed_npubs_write
- Write operations: Publish, Delete, Create, Update, Insert, Remove, Set, Put
- Read operations: everything else (Query, Subscribe, Get, List, etc.)
- Auth interceptor checks appropriate list based on method type
- Enables common patterns:
  - Public relay: only some can write, everyone can read
  - Private relay: restricted read and write
  - Open relay: everyone can read and write
- Updated config, docs, and comprehensive tests

Use cases: "only some can write, everyone can read"

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