Restructure into src/tests and add build foundation (M0)

Moves the scaffold to src/DodoSSH.Api and establishes the repo conventions the rest
of the milestones build on.

Structure:
- src/{Contracts,Crypto,Domain,Infrastructure,Api}, tests/{Contracts,Crypto,Domain}.Tests
- DodoSSH.slnx rewritten with src/ and tests/ solution folders

Build:
- Directory.Build.props centralises TFM, nullable, deterministic builds and
  TreatWarningsAsErrors; Directory.Packages.props pins every version centrally
- packages.lock.json committed so CI restores in locked mode
- NuGet.config clears machine-level sources, which both fixes NU1507 under central
  package management and makes restore reproducible off this machine
- Microsoft.OpenApi pinned to 2.11.0: ASP.NET Core 10.0.10 resolves 2.0.0, which is
  covered by GHSA-v5pm-xwqc-g5wc (high, patched in 2.7.5)

Analyzers:
- AnalysisLevel is Recommended, not All. With warnings-as-errors, All turns opinionated
  naming rules into build breaks and trains people to blanket-suppress.
- BannedSymbols.txt bans DateTime.UtcNow (TimeProvider), Guid.NewGuid (CreateVersion7),
  sync-over-async, MD5/SHA1, PBKDF2 and SecureString
- CA1711/CA1724 disabled: both are .NET Framework CAS-era naming rules
- PublicApiAnalyzers on Contracts only, since that assembly is the client's real contract

API:
- weather-forecast template removed
- UseHttpsRedirection removed; TLS terminates at the reverse proxy and redirecting
  behind one causes loops
- /healthz/{live,ready,startup}. Liveness deliberately checks no dependencies so a
  transient database outage cannot restart the container and kill live SSH sessions.

Notes:
- No coverage collector yet. Microsoft.Testing.Extensions.CodeCoverage pulls an MTP 1.x
  MSBuild extension that throws TypeLoadException against the MTP 2.3.x xunit.v3 brings.
  Coverage gates are an M3 concern; revisit with an MTP 2.x-aligned version then.

Verified: dotnet build (0 warnings), 17 tests pass, format check clean, API serves
health and OpenAPI endpoints.

src/DodoSSH.Api/Setup/OpenApi.cs

@@ -0,0 +1,27 @@
+namespace DodoSSH.Api.Setup;
+
+/// <summary>
+/// OpenAPI document configuration.
+/// </summary>
+/// <remarks>
+/// The generated document exists for third parties and a future CLI. It is emitted at
+/// build time to <c>artifacts/openapi/v1.json</c> and diffed in CI so an unintended
+/// contract change fails the pull request. The desktop client's actual contract is the
+/// <c>DodoSSH.Contracts</c> assembly, guarded by PublicApiAnalyzers.
+/// </remarks>
+internal static class OpenApi
+{
+    internal const string DocumentName = "v1";
+
+    internal static IServiceCollection AddDodoOpenApi(this IServiceCollection services)
+    {
+        services.AddOpenApi(DocumentName);
+
+        // Added in M1, once there are endpoints to describe:
+        //   - a document transformer contributing the OAuth2 authorizationCode + PKCE
+        //     security scheme, so the document is usable from a generated client
+        //   - a schema transformer mapping byte[] to {type: string, format: byte},
+        //     since every ciphertext field crosses the wire as base64
+        return services;
+    }
+}