drop Yavsc.Web references from Dockerfiles, ROADMAP, arch doc

Yavsc.Web is an empty template/draft project — no useful code,
conceptually a duplicate of the front that lives in Yavsc.Org.
Remove its mentions from:

- Dockerfile, Dockerfile.backend: the COPY src/Yavsc.Web/*.csproj
  step is useless (the project is referenced nowhere downstream)
- ROADMAP.md: the 'Perimetre technique' table no longer lists it
- doc/architecture/decoupage-organisation.md: removed from the
  ASCII diagram and the per-project table

Note: this commit only removes references; the empty project
itself (src/Yavsc.Web/) and the yavsc.sln Project() entry are
left in place for now. A future commit can rm -rf the directory
and prune the .sln when we're sure nothing else still depends
on it.

ROADMAP.md

@@ -35,7 +35,7 @@ Trois principes non négociables traversent tous les jalons :
 
 | Brique | Module | État |
 |---|---|---|
-| Backend ASP.NET | `Yavsc.Server`, `Yavsc.Web`, `Yavsc.Api`, `Yavsc.Org` | actif, build OK, cible .NET 10 |
+| Backend ASP.NET | `Yavsc.Server`, `Yavsc.Api`, `Yavsc.Org` | actif, build OK, cible .NET 10 |
 | Domaine partagé | `Yavsc.Abstract` | actif |
 | Blogs | `Yavsc.Blogs` | actif |
 | Client chat (`PostIt`) | `PostIt` (Avalonia) + `PostIt.Android` / `PostIt.Browser` / `PostIt.Desktop` | actif, OIDC |