Add activitives.platform to some cubejs pre-aggregations

[sausage-todd]

Changes proposed ✍️

What

🤖 Generated by Copilot at ade62bf

Added a new dimension to track the platform of member activities, and moved and enriched some segments from the Organizations cube to the Members cube. This allows for better platform analysis of organization growth and engagement.
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🤖 Generated by Copilot at ade62bf

Sing, O Muse, of the skillful coder who devised
A new dimension for the Members cube, to reveal
The varied platforms of their deeds, whether web or mobile,
And thus illuminate their habits and their zeal.

Why

How

🤖 Generated by Copilot at ade62bf

• Add Activities.platform dimension to Members cube to enable filtering and grouping by activity source (link, link, link, link) in backend/src/cubejs/schema/Members.js

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.

[epipav]

lgtm 👍 Are the numbers correct with Activities.platform added to preaggs? Sometimes with one-many relationship it causes row duplication when hit by preaggs (meaning it might count same member twice when we include it in the preaggregation and when the member have multiple platform activities)