feat(http): implement shared Bearer auth middleware (resolve_from_token, stash Identity in request extensions)

Add src/server/auth.rs with bearer_auth_middleware axum layer that
extracts the Authorization: Bearer header, resolves via
IdentityProvider::resolve_from_token, and stashes Option<Identity> in
request extensions. Shared by HTTP gateway routes and the to_mcp rmcp
service (research §4.4). No token, malformed header, or failed
resolution all yield None (unauthenticated, not an error) — Bearer-only
auth mechanism (ADR-004).

Includes ResolvedIdentity axum extractor reading from extensions, and
wires the middleware into the HttpAdapter router around the
gateway/openapi/mcp routes (excluding the raw /healthz route).

crates/alknet-http/Cargo.toml

@@ -42,4 +42,7 @@ rmcp = { version = "1.8", optional = true, default-features = false, features =
     "server",
     "transport-streamable-http-client-reqwest",
     "transport-streamable-http-server",
-] }
+] }
+
+[dev-dependencies]
+tower = "0.5"