Make `Boot` a freestanding app type, rather than a view of `crate::app::App`'s internals.

In the course of working on web push, I determined that we probably need to make `App` generic over the web push client we're using, so that tests can use a dummy client while the real app uses a client created at startup and maintained over the life of the program's execution. The most direct implementation of that is to render App as `App<P>`, where the parameter is occupied by the specific web push client type in use. However, doing this requires refactoring at _every_ site that mentions `App`, including every handler, even though the vast majority of those sites will not be concerned with web push.

I reviewed a few options with @wlonk:

* Accept the type parameter and apply it everywhere, as the cost of supporting web push.
* Hard-code the use of a specific web push client.
* Insulate handlers &c from `App` via provider traits, mimicing what we do for repository provider traits today.
* Treat each app type as a freestanding state in its own right, so that only push-related components need to consider push clients (as far as is feasible).

This is a prototype towards that last point, using a simple app component (boot) as a testbed. `FromRef` allows handlers that take a `Boot` to be used in routes that provide an `App`, so this is a contained change. However, the structure of `FromRef` prevents `Boot` from carrying any lifetime narrower than `'static`, so it now holds clones of the state fields it acquires from App, instead of references. This is fine - that's just a database pool, and sqlx's pool type is designed to be shared via cloning. From <https://docs.rs/sqlx/latest/sqlx/struct.Pool.html>:

> Cloning Pool is cheap as it is simply a reference-counted handle to the inner pool state.

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