Add configuration, OIDC auth wiring and discovery endpoints (M1)

Options, JWT bearer validation, the /meta and .well-known endpoints, and a dev compose
stack with Keycloak. Verified end to end: compose up, migrate, run, both discovery
endpoints return correct payloads, and readiness reports the schema current.

Configuration:
- Strongly-typed options for Server, Oidc, Relay and Sync, all ValidateOnStart. A
  self-hosted server that boots half-configured and fails later per-request is far harder
  to diagnose than one that refuses to start and names the bad setting.
- Cross-field validation the annotations cannot express: relay needs a WebSocketUrl when
  enabled, idle timeout must be under max session duration, item payload cap under batch cap.
- Startup warnings for combinations that are individually valid but dangerous together:
  RequireHttpsMetadata false outside Development, and AllowEmailLinking (which turns any
  token bearing a victim's email into account takeover, hence default false).

Auth:
- JwtBearer with ClockSkew cut to 30s from the 5-minute default; five minutes of slack on a
  credential granting vault ciphertext access is more than any clock needs.
- IncludeErrorDetails off, and a FallbackPolicy so an endpoint without an explicit policy
  still requires a caller rather than silently being public.

Discovery, per ADR 0002:
- /api/v1/meta reports versions, features and push caps.
- /.well-known/dodossh-configuration is the onboarding story: the user types one server URL
  and the client discovers OIDC authority, client id, scopes and relay endpoint.

Two environment problems found by actually running the stack:
- PostgreSQL 18 changed its data mount point. Mounting /var/lib/postgresql/data — correct
  through 17 — makes the image refuse to start; 18+ wants a single mount at
  /var/lib/postgresql with the cluster in a subdirectory.
- Keycloak moved to host port 18080. An unrelated Apache Tomcat on this machine holds
  127.0.0.1:8080, and a loopback-specific bind beats Docker's 0.0.0.0 publish for
  "localhost". It presents as Keycloak 404ing every realm while its own log says the import
  succeeded, which is a genuinely misleading failure.

Also: CA1848 is enforced, not advisory — warnings are errors, so the .editorconfig comment
claiming otherwise was wrong. Startup and health logging now uses [LoggerMessage]. And a
clean rebuild is back to zero warnings; the incremental build had been hiding 40 in test
projects (banned Guid.NewGuid, an obsolete Testcontainers constructor, and two analyzer
families that are genuinely noise under a test host).

Verified: 0 warnings on a clean rebuild, 122 tests pass, format clean.

src/DodoSSH.Api/Setup/Persistence.cs

@@ -0,0 +1,85 @@
+using DodoSSH.Infrastructure;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Diagnostics.HealthChecks;
+
+namespace DodoSSH.Api.Setup;
+
+/// <summary>Database wiring.</summary>
+internal static class Persistence
+{
+    private const string ConnectionStringName = "Postgres";
+
+    internal static IServiceCollection AddDodoPersistence(
+        this IServiceCollection services,
+        IConfiguration configuration)
+    {
+        var connectionString = configuration.GetConnectionString(ConnectionStringName)
+            ?? throw new InvalidOperationException(
+                $"ConnectionStrings:{ConnectionStringName} is not configured.");
+
+        services.AddDbContext<DodoDbContext>(options => options
+            .UseNpgsql(connectionString, npgsql => npgsql
+                .MigrationsHistoryTable("__EFMigrationsHistory", DodoDbContext.SchemaName)
+                // Retry on transient faults only. Note this is safe here because writes go
+                // through explicitly-managed transactions rather than relying on the execution
+                // strategy to replay ambient ones.
+                .EnableRetryOnFailure(3))
+            .UseSnakeCaseNamingConvention());
+
+        services.AddHealthChecks()
+            .AddCheck<DatabaseHealthCheck>(
+                "postgres",
+                tags: [HealthChecks.ReadyTag, HealthChecks.StartupTag]);
+
+        return services;
+    }
+}
+
+/// <summary>
+/// Reports the database reachable and the schema current.
+/// </summary>
+/// <remarks>
+/// Pending migrations make the instance unready rather than crashing it. The API never migrates in
+/// production — a separate migrator job does — so the correct response to a schema mismatch is to
+/// stop taking traffic and say so loudly, not to attempt a repair.
+/// </remarks>
+internal sealed class DatabaseHealthCheck(DodoDbContext context, ILogger<DatabaseHealthCheck> logger)
+    : IHealthCheck
+{
+    /// <inheritdoc />
+    public async Task<HealthCheckResult> CheckHealthAsync(
+        HealthCheckContext healthCheckContext,
+        CancellationToken cancellationToken = default)
+    {
+        try
+        {
+            if (!await context.Database.CanConnectAsync(cancellationToken).ConfigureAwait(false))
+            {
+                return HealthCheckResult.Unhealthy("Cannot connect to PostgreSQL.");
+            }
+
+            var pending = await context.Database
+                .GetPendingMigrationsAsync(cancellationToken)
+                .ConfigureAwait(false);
+
+            var pendingList = pending.ToList();
+            if (pendingList.Count > 0)
+            {
+                // Computed once and used for both the log and the health result, so this is not
+                // work done only for logging.
+                var pendingDescription = string.Join(", ", pendingList);
+
+                StartupLog.PendingMigrations(logger, pendingList.Count, pendingDescription);
+
+                return HealthCheckResult.Unhealthy(
+                    $"{pendingList.Count} migration(s) pending: {pendingDescription}");
+            }
+
+            return HealthCheckResult.Healthy();
+        }
+        catch (Exception exception) when (exception is not OperationCanceledException)
+        {
+            return HealthCheckResult.Unhealthy("PostgreSQL health check failed.", exception);
+        }
+    }
+}