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/StartupLog.cs

@@ -0,0 +1,38 @@
+namespace DodoSSH.Api.Setup;
+
+/// <summary>
+/// Source-generated log messages for startup and health.
+/// </summary>
+/// <remarks>
+/// <c>[LoggerMessage]</c> rather than <c>ILogger</c> extension calls: allocation-free, and the
+/// event ids and message templates become a stable, greppable inventory instead of string literals
+/// scattered through wiring code.
+/// </remarks>
+internal static partial class StartupLog
+{
+    [LoggerMessage(
+        EventId = 1001,
+        Level = LogLevel.Warning,
+        Message = "Oidc:RequireHttpsMetadata is false outside Development. Token signing keys are "
+            + "being fetched over plaintext HTTP, so anyone on the network path can serve their "
+            + "own keys and mint tokens this server will accept.")]
+    internal static partial void InsecureMetadataOutsideDevelopment(ILogger logger);
+
+    [LoggerMessage(
+        EventId = 1002,
+        Level = LogLevel.Warning,
+        Message = "Oidc:AllowEmailLinking is enabled. A token from any configured provider "
+            + "carrying a victim's email address will be linked to that victim's existing account. "
+            + "Only enable this when every configured provider verifies email ownership.")]
+    internal static partial void EmailLinkingEnabled(ILogger logger);
+
+    [LoggerMessage(
+        EventId = 1010,
+        Level = LogLevel.Critical,
+        Message = "Database schema is out of date: {PendingCount} migration(s) pending "
+            + "({PendingMigrations}). Readiness will fail until the migrator has run.")]
+    internal static partial void PendingMigrations(
+        ILogger logger,
+        int pendingCount,
+        string pendingMigrations);
+}