Implement test sharding with @shard decorator for sequential vs async

Summary

Implement test sharding with @shard(async=True/False) decorator. Context is sqlite concurrency downside solved by per-customer sqlite DBs as sharding, applied to tests where concurrent testing breaks with data-dependent tests. Design is two shards: one sequential for tests that contain data, other extremely async (e.g. 48 playwright browsers at once on 64GB machine). DevPlace has 4000 tests with no sharding yet; reference project with 1000 tests shows it matters. Tasks are design shard decorator, split runner into sequential vs async shards, update CI config, measure speedup.

Steps to Reproduce

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Expected Behaviour

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Actual Behaviour

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Environment

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Reported by retoor via DevPlace.