fix(profile): expose volume_size_gb on Profile model

[ClaydeCode]

Summary

• Add volume_size_gb: int | None to the Profile model and propagate it from ShardBase in Profile.from_shard.
• Extend tests/test_profile.py with a case asserting the field round-trips through GET /protected/management/profile.

Why this matters

This is a prerequisite for web-terminal#20 (Subscription card on the Settings page). The Trial-state "Subscribe — €X/month" button label needs volume_size_gb to apply the pricing formula from landing-page/src/components/Pricing.astro (disk price per GB × volume size). Without this field, the web terminal can only render a placeholder for the price, producing a degraded UX during the initial billing rollout. ShardBase already carries the field, so this is purely a model-surface fix on the shard side.

Recommended reading order

1. shard_core/data_model/profile.py — the model + from_shard change.
2. tests/conftest.py — mock_shard gains volume_size_gb=30 so the propagation has something to assert against.
3. tests/test_profile.py — new test_profile_includes_volume_size_gb.

Test plan

• test_profile still passes (existing model-equality assertion).
• test_profile_includes_subscription still passes.
• test_profile_includes_volume_size_gb — new: asserts Profile.volume_size_gb == mock_shard.volume_size_gb == 30.
• test_signed_call (uses Profile.from_shard) still passes.
• just cleanup (ruff + black on shard_core and tests) clean.
• Manual: after merging, web-terminal#20 reads $store.state.profile.volume_size_gb and renders the Subscribe price label.

Notes

• from_shard uses getattr(shard, "volume_size_gb", None) for safety against runtime subclasses, mirroring the existing subscription field.
• No DB migration needed — Profile is stored as JSON via set_value in kv_store, and the field defaults to None.

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