Decouple `feder-server` from Axum with a Tower-compatible HTTP interface

[sij411]

Currently, feder-server is tightly coupled with Axum, a web framework. The goal of this issue is to make the server framework-agnostic so that Axum becomes only one possible adapter.

Requirments

1. Remove Axum dependencies from feder-server

   • feder-server must not depend on axum::Router, Axum request/response types, extractors, IntoResponse, or other Axum-specific APIs.
   • Axum-specific integration should move into a separate adapter such as feder-axum.

2. Introduce a reusable HTTP boundary

   • Shared HTTP request/response types and ActivityPub-over-HTTP behaviour should live in feder-http.
   • feder-server should consume and produce these HTTP types rather than Axum-specific ones.
   • Prefer standard http crate types where practical.

3. Make feder-server Tower-compatible

   • Tower should only define the request/response processing contract; it should not dictate the request and response types themselves.
   • feder-server should expose a tower_service::Service interface.

Service<FederRequest, Response = FederResponse>

4. Move Axum integration into feder-axum

   • feder-axum should handle routing, extractors, Axum body conversion, and conversion between Axum and feder-http.
   • Applications should be able to combine feder-server with feder-axum explicitly.

application
    ↓
feder-axum
    ↓
feder-server
    ↓
feder-http / feder-core

Notes

• The design should allow another adapter, such as an embedded HTTP runtime, to use feder-server without depending on Axum.
• Embedded support does not need to be implemented in this issue; the architecture only needs to avoid preventing it.