Cubejs membersByActivityPlatform preagg also includes members.joinedAtUnixTs dimension

[epipav]

Changes proposed ✍️

What

🤖 Generated by Copilot at 0f0f5b2

This pull request adds the joinedAtUnixTs member to the Members cube schema in backend/src/cubejs/schema/Members.js. This allows querying the members by their join date in Unix timestamp format.
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🤖 Generated by Copilot at 0f0f5b2

Members cube grows
joinedAtUnixTs appears
Query by seasons

Why

How

🤖 Generated by Copilot at 0f0f5b2

• Add joinedAtUnixTs member to Members cube schema to enable querying by join date in Unix timestamp format (link)

Checklist ✅

• Label appropriately with Feature, Improvement, or Bug.
• Add screehshots to the PR description for relevant FE changes
• New backend functionality has been unit-tested.
• API documentation has been updated (if necessary) (see docs on API documentation).
• Quality standards are met.