chore: move the server, services and tools to .NET 10

.NET 8 and 9 reach end of support on 2026-11-10; .NET 10 is the LTS release
(supported until November 2028).

- net9.0 -> net10.0 for every server, service, gateway, test and tool project
- ASP.NET Core / EF Core / SignalR Redis 10.0.13, Npgsql EF 10.0.3,
  Serilog.AspNetCore 10.0.0
- Dockerfile / Dockerfile.service: sdk and aspnet 10.0 images
- release workflow: setup-dotnet 10.0.x
- Jenkins build agent: dotnet-sdk-10.0 (was 8.0); the ClientMod still
  targets net6.0, which MelonLoader loads
- README / architecture docs, .gitignore data paths

DorkNet.AdminMobile (MAUI) stays on net9.0 for now: building it needs the
Android/iOS workloads.

README.md

@@ -64,7 +64,7 @@ What the 2023 client gets here that the 2020 branches don't have:
 
 ## Quickstart
 
-**1. Install .NET 9 SDK** and (optionally) `dotnet-ef`:
+**1. Install .NET 10 SDK** and (optionally) `dotnet-ef`:
 
 ```pwsh
 dotnet tool install --global dotnet-ef
@@ -101,7 +101,7 @@ cd DorkNet
 dotnet run --project DorkNet.Server
 ```
 
-First boot creates `bin/Debug/net9.0/data/dorknet.db`, runs every
+First boot creates `bin/Debug/net10.0/data/dorknet.db`, runs every
 `LegacyUpgrades` pass, seeds canonical rooms / store catalog / club
 categories / playlists, and starts listening. You should see
 `Application started.` in the log; `curl http://localhost:8080/healthz`