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.

deploy/keycloak/realm-dodossh.json

@@ -0,0 +1,81 @@
+{
+  "realm": "dodossh",
+  "enabled": true,
+  "displayName": "DodoSSH (development)",
+  "sslRequired": "none",
+  "registrationAllowed": false,
+  "loginWithEmailAllowed": true,
+  "duplicateEmailsAllowed": false,
+  "accessTokenLifespan": 900,
+  "ssoSessionIdleTimeout": 1800,
+  "ssoSessionMaxLifespan": 36000,
+
+  "clients": [
+    {
+      "clientId": "dodossh-desktop",
+      "name": "DodoSSH Desktop",
+      "enabled": true,
+      "protocol": "openid-connect",
+      "publicClient": true,
+      "standardFlowEnabled": true,
+      "directAccessGrantsEnabled": false,
+      "serviceAccountsEnabled": false,
+      "implicitFlowEnabled": false,
+      "attributes": {
+        "pkce.code.challenge.method": "S256",
+        "post.logout.redirect.uris": "http://127.0.0.1:*/*"
+      },
+      "redirectUris": [
+        "http://127.0.0.1:*/callback",
+        "http://localhost:*/callback"
+      ],
+      "webOrigins": [],
+      "protocolMappers": [
+        {
+          "name": "dodossh-api-audience",
+          "protocol": "openid-connect",
+          "protocolMapper": "oidc-audience-mapper",
+          "consentRequired": false,
+          "config": {
+            "included.custom.audience": "dodossh-api",
+            "access.token.claim": "true",
+            "id.token.claim": "false"
+          }
+        }
+      ]
+    }
+  ],
+
+  "users": [
+    {
+      "username": "alice",
+      "enabled": true,
+      "emailVerified": true,
+      "email": "alice@example.com",
+      "firstName": "Alice",
+      "lastName": "Example",
+      "credentials": [
+        {
+          "type": "password",
+          "value": "alice",
+          "temporary": false
+        }
+      ]
+    },
+    {
+      "username": "bob",
+      "enabled": true,
+      "emailVerified": true,
+      "email": "bob@example.com",
+      "firstName": "Bob",
+      "lastName": "Example",
+      "credentials": [
+        {
+          "type": "password",
+          "value": "bob",
+          "temporary": false
+        }
+      ]
+    }
+  ]
+}