Clarify and rename `firstActiveDay` semantics

Nobody has claimed this yet.

We added a new [firstActiveDay](https://github.com/livepeer/subgraph/blob/25b5e5fc01fcf92f59eb473a51f101ef93af856b/schema.graphql#L247) field to the Broadcaster entity in https://github.com/livepeer/subgraph/pull/168. However due to a patch we had to do in https://github.com/livepeer/subgraph/pull/202#issuecomment-3745212017 this now is a bit ambiguous because [activeBroadcasters](https://github.com/livepeer/subgraph/blob/25b5e5fc01fcf92f59eb473a51f101ef93af856b/schema.graphql#L58) on the Protocol entity only tracks broadcasters that are actively sending fees. We might consider deprecating `firstActiveDay` in favor of a clearer field such as `activationTimestap` and `lastActiveDay` to `lastActiveTimestamp`, and update the schema and related logic accordingly.

Example

```graphql
"The date this broadcaster first funded a deposit or reserve, beginning at 12:00am UTC"
firstFundedTimestamp: Int!
"The date this broadcaster first funded a deposit or reserve, beginning at 12:00am UTC"
firstActiveDay: Int! @deprecated(reason: "Use firstFundedDay instead.")
```

Start with the Broadcaster and Protocol definitions in schema.graphql, especially the referenced lines around firstActiveDay and activeBroadcasters, then read the linked pull request context. The work is done when the field semantics and names are unambiguous and the schema plus related logic consistently reflect the agreed changes.

Written by the indexing model from the issue text.