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.

.editorconfig

@@ -98,9 +98,10 @@ dotnet_diagnostic.IDE0055.severity = error
 # meaningful in the Avalonia client, which re-enables CA2007 in its own .editorconfig.
 dotnet_diagnostic.CA2007.severity = none
 
-# Prefer LoggerMessage source generation over ILogger extension calls — allocation-free
-# and gives structured events by construction. Warning, so it is visible but not a wall
-# during early development; raised to error once the logging pass lands in M4.
+# Require LoggerMessage source generation over ILogger extension calls — allocation-free,
+# and event ids plus message templates become a greppable inventory rather than string
+# literals scattered through the code. Effectively an error, since warnings are errors;
+# stated as such rather than pretending it is advisory.
 dotnet_diagnostic.CA1848.severity = warning
 
 # Exceptions carry ProblemDetails codes, not localised text.