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.

Directory.Build.props

@@ -0,0 +1,51 @@
+<Project>
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>latest</LangVersion>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <PropertyGroup Label="Quality gates">
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
+    <!--
+      Recommended, not All. With TreatWarningsAsErrors on, latest-All turns opinionated
+      naming rules (CA1711 reserved suffixes, CA1724 namespace collisions) into build
+      breaks, which trains people to reach for blanket suppressions and devalues the
+      analyzers that catch real defects. Correctness and security rules we specifically
+      want beyond Recommended are raised individually in .editorconfig.
+    -->
+    <AnalysisLevel>latest-Recommended</AnalysisLevel>
+    <AnalysisMode>Recommended</AnalysisMode>
+    <!-- The whole product is UTC-only and container-hosted; no tzdata, no culture-sensitive
+         formatting. Set here rather than per-project so it cannot drift. -->
+    <InvariantGlobalization>true</InvariantGlobalization>
+  </PropertyGroup>
+
+  <PropertyGroup Label="Reproducible builds">
+    <Deterministic>true</Deterministic>
+    <ContinuousIntegrationBuild Condition="'$(CI)' == 'true'">true</ContinuousIntegrationBuild>
+    <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
+    <CentralPackageTransitivePinningEnabled>true</CentralPackageTransitivePinningEnabled>
+    <!-- packages.lock.json is committed; CI restores in locked mode. -->
+    <RestorePackagesWithLockFile>true</RestorePackagesWithLockFile>
+  </PropertyGroup>
+
+  <PropertyGroup Label="Assembly metadata">
+    <Company>DodoTech</Company>
+    <Product>DodoSSH</Product>
+    <NeutralLanguage>en</NeutralLanguage>
+  </PropertyGroup>
+
+  <ItemGroup Label="Analyzers">
+    <PackageReference Include="Microsoft.CodeAnalysis.BannedApiAnalyzers" PrivateAssets="all" />
+    <PackageReference Include="Meziantou.Analyzer" PrivateAssets="all" />
+  </ItemGroup>
+
+  <ItemGroup Label="Banned API list">
+    <AdditionalFiles Include="$(MSBuildThisFileDirectory)BannedSymbols.txt" Visible="false" />
+  </ItemGroup>
+
+</Project>