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/Configuration.cs

@@ -0,0 +1,74 @@
+using Microsoft.Extensions.Options;
+
+namespace DodoSSH.Api.Setup;
+
+/// <summary>Binds and validates strongly-typed options.</summary>
+/// <remarks>
+/// Every section uses <c>ValidateOnStart</c>. 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 says
+/// which setting is wrong.
+/// </remarks>
+internal static class Configuration
+{
+    internal static IServiceCollection AddDodoOptions(this IServiceCollection services)
+    {
+        services.AddOptions<ServerOptions>()
+            .BindConfiguration(ServerOptions.SectionName)
+            .ValidateDataAnnotations()
+            .ValidateOnStart();
+
+        services.AddOptions<OidcOptions>()
+            .BindConfiguration(OidcOptions.SectionName)
+            .ValidateDataAnnotations()
+            .ValidateOnStart();
+
+        services.AddOptions<SyncOptions>()
+            .BindConfiguration(SyncOptions.SectionName)
+            .ValidateDataAnnotations()
+            .Validate(
+                options => options.MaxItemPayloadBytes <= options.MaxPayloadBytes,
+                "Sync:MaxItemPayloadBytes must not exceed Sync:MaxPayloadBytes.")
+            .Validate(
+                options => options.DefaultPullLimit <= options.MaxPullLimit,
+                "Sync:DefaultPullLimit must not exceed Sync:MaxPullLimit.")
+            .ValidateOnStart();
+
+        services.AddOptions<RelayOptions>()
+            .BindConfiguration(RelayOptions.SectionName)
+            .ValidateDataAnnotations()
+            .Validate(
+                options => !options.Enabled || !string.IsNullOrWhiteSpace(options.WebSocketUrl),
+                "Relay:WebSocketUrl is required when Relay:Enabled is true.")
+            .Validate(
+                options => options.IdleTimeout < options.MaxSessionDuration,
+                "Relay:IdleTimeout must be shorter than Relay:MaxSessionDuration.")
+            .ValidateOnStart();
+
+        return services;
+    }
+
+    /// <summary>
+    /// Warns about configuration that is individually valid but dangerous in combination.
+    /// </summary>
+    /// <remarks>
+    /// These cannot be hard failures — each is legitimate in some deployment — but silently
+    /// accepting them produces subtly broken security properties that nobody notices.
+    /// </remarks>
+    internal static WebApplication WarnOnRiskyConfiguration(this WebApplication app)
+    {
+        var logger = app.Services.GetRequiredService<ILoggerFactory>().CreateLogger("DodoSSH.Startup");
+        var oidc = app.Services.GetRequiredService<IOptions<OidcOptions>>().Value;
+
+        if (!oidc.RequireHttpsMetadata && !app.Environment.IsDevelopment())
+        {
+            StartupLog.InsecureMetadataOutsideDevelopment(logger);
+        }
+
+        if (oidc.AllowEmailLinking)
+        {
+            StartupLog.EmailLinkingEnabled(logger);
+        }
+
+        return app;
+    }
+}