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feat: Add lifecycle summary info read model + average time spent in lifecycle query (#8691)

This PR adds a project lifecycle read model file along with the most
important (and most complicated) query that runs with it: calculating
the average time spent in each stage.

The calculation relies on the following:
- when calculating the average of a stage, only flags who have gone into
a following stage are taken into account.
- we'll count "next stage" as the next row for the same feature where
the `created_at` timestamp is higher than the current row
- if you skip a stage (go straight to live or archived, for instance),
that doesn't matter, because we don't look at that.

The UI only shows the time spent in days, so I decided to go with
rounding to days directly in the query.

## Discussion point:

This one uses a subquery, but I'm not sure it's possible to do without
it. However, if it's too expensive, we can probably also cache the value
somehow, so it's not calculated more than every so often.

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