fix: Use token.id for ApiToken outbox sharding

[akramcodez]

Summary

Fixes the stampeding lock problem where Sentry App tokens share the same proxy_user_id as their outbox shard_identifier, causing severe lock contention.

Closes #105425

Changes

• Override outboxes_for_update() on ApiToken to use token.id as shard_identifier
• Add tests to verify sharding behavior

Before vs After

Scenario	Before (user_id)	After (token.id)
Sentry App with 10K installations	1 shard, all serialized	10K shards, fully parallel
User with 5 tokens	1 shard, serialized	5 shards, parallel

Testing

• Added unit tests for outbox sharding behavior
• Verified Sentry App tokens with same proxy_user get different shards
• Verified explicit shard_identifier override still works