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.Domain/Authorization/PermissionFlags.cs

@@ -0,0 +1,41 @@
+namespace DodoSSH.Domain.Authorization;
+
+/// <summary>
+/// PermissionFlags a subject (user or team) may hold over a vault or an individual resource.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Evaluation is a plain union across the subject's direct grants and the grants held by
+/// teams they belong to. There are deliberately no Deny rules: union-only evaluation is
+/// monotonic and straightforward to test, and Deny can be added later additively if a
+/// real need appears. Express restriction by granting narrowly instead.
+/// </para>
+/// <para>
+/// <see cref="Connect"/> is a user-interface hint, <b>not</b> a security boundary. SSH
+/// terminates on the client, so opening a session requires the credential's plaintext on
+/// that machine; "may connect but may not view the key" is therefore unenforceable in
+/// this architecture. Treat it as an anti-shoulder-surfing convenience and never document
+/// it as access control. See docs/adr/0001-e2ee-trust-model.md.
+/// </para>
+/// </remarks>
+[Flags]
+public enum PermissionFlags
+{
+    /// <summary>No access.</summary>
+    None = 0,
+
+    /// <summary>May fetch and decrypt the resource. Implies the ability to use it.</summary>
+    Read = 1 << 0,
+
+    /// <summary>May create, modify and soft-delete resources in the vault.</summary>
+    Write = 1 << 1,
+
+    /// <summary>Intent hint that the subject uses this host for sessions. Not a boundary.</summary>
+    Connect = 1 << 2,
+
+    /// <summary>May grant access to other subjects, which requires re-wrapping the vault key.</summary>
+    Share = 1 << 3,
+
+    /// <summary>May administer the vault itself: rename, rekey, manage ACLs.</summary>
+    Admin = 1 << 4,
+}

src/DodoSSH.Domain/DodoSSH.Domain.csproj

@@ -0,0 +1,12 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <!--
+    Entities, enums and invariants. No EF Core reference: persistence concerns live in
+    DodoSSH.Infrastructure so the domain stays unit-testable with no database.
+  -->
+
+  <ItemGroup>
+    <InternalsVisibleTo Include="DodoSSH.Domain.Tests" />
+  </ItemGroup>
+
+</Project>

src/DodoSSH.Domain/packages.lock.json

@@ -0,0 +1,19 @@
+{
+  "version": 2,
+  "dependencies": {
+    "net10.0": {
+      "Meziantou.Analyzer": {
+        "type": "Direct",
+        "requested": "[3.0.134, )",
+        "resolved": "3.0.134",
+        "contentHash": "tTYCcYKyOko3TMNxmxmA9nakbcHVUgglENmCMIhzIjl9y9FBZO/0tWSxTGC74Sp198FmWih5S5KkjQRBg5ePkQ=="
+      },
+      "Microsoft.CodeAnalysis.BannedApiAnalyzers": {
+        "type": "Direct",
+        "requested": "[5.6.0, )",
+        "resolved": "5.6.0",
+        "contentHash": "Kcobt3pnOdO0A+6CKiMHZdTEluJpsfxiV20axtZdmfBQnDmiWTKPJADlgAfdTuKNAnVarrkJa0UEGwuOo91muw=="
+      }
+    }
+  }
+}