Add --min-storage and --max-storage flags to database and branch creation

[nholden]

Summary

Users running automated import workflows need to configure storage size at database creation time. Without this, they have to create a database with the default minimum storage and immediately resize — often hitting storage limits before the resize takes effect (planetscale/surfaces#3485).

The API now supports storage configuration at creation time (planetscale/api-bb#18339, planetscale/api-bb#18389), and the SDK has been updated to match (planetscale/planetscale-go#303). This PR exposes those parameters in the CLI.

Usage

# Create a PostgreSQL database with storage configuration
pscale database create mydb --engine postgresql --min-storage 10737418240 --max-storage 107374182400

# Create a branch with storage configuration
pscale branch create mydb mybranch --min-storage 10737418240 --max-storage 107374182400

Flags accept bytes, consistent with existing CLI conventions (e.g., --max-query-size). Storage flags are only supported for PostgreSQL — the CLI returns a clear error if used with a MySQL database or branch.

Blocked on: a new release of planetscale-go after planetscale/planetscale-go#303 is merged. go.mod will need to be updated to the new version before this can pass CI.

Test plan

• TestDatabase_CreateCmdWithStorage
• TestDatabase_CreateCmdWithStorageMySQLError
• TestBranch_CreateCmdWithStorage
• TestBranch_CreateCmdWithStorageMySQLError
• Manually verified: created a PostgreSQL database and branch with storage flags
• Manually verified: MySQL database and branch creation correctly rejected

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