Move OIDC config to appsettings.json and add MetadataAddress

Authority, Audience, and MetadataAddress are not secrets so they belong
in committed config rather than runtime env vars. MetadataAddress points
to the internal Docker URL for JWKS fetch, avoiding nginx hairpinning;
it is blanked in Development so the JWT middleware falls back to
Authority-based discovery. RequireHttpsMetadata is disabled only when
MetadataAddress is set (internal http URL).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -5,10 +5,6 @@ POSTGRES_DB=budget
 POSTGRES_USER=budget
 POSTGRES_PASSWORD=changeme
 
-# Auth
-AUTH__AUTHORITY=https://auth.stwaddle.com
-AUTH__AUDIENCE=budget_api
-
 # Client (baked into Vite build)
 VITE_AUTH_AUTHORITY=https://auth.stwaddle.com
 VITE_AUTH_CLIENT_ID=budget-client