Wrapping middleware #2: Validation

[LukeMathWalker]

The registered middlewares are now properly validation by pavexc:

• they take Next as input (once)
• they return a type that can be converted into a Response (either directly or in the happy case)
• they don't use generic parameters that we can't reliably infer

I did some refactorings along the way, especially in the component database (arguably the messiest part of the codebase).