feat(login): publish verify key as JWKS

Auth serves its Ed25519 public key at GET /.well-known/jwks.json, so
Core can find the key by the token kid. Startup fails on a blank
JWT_KID, because compose turns an unset value into "" and Core finds
keys only by kid.

readme.md

@@ -18,7 +18,8 @@ To Be Played is split into three repositories:
   block for 15 minutes with `429 TOO_MANY_ATTEMPTS` and `Retry-After`. Unknown emails cost the same BCrypt time as a
   wrong password, and every failed login gets the same error.
 - JWT signed with **EdDSA (Ed25519)**. Auth holds the private key, Core only the public key, so Core can verify
-  tokens but never create them. Tokens carry a `kid` header for key rotation.
+  tokens but never create them. Tokens carry a `kid` header, and `GET /.well-known/jwks.json` publishes the
+  public key as a JWK, so Core finds the right key after a rotation.
 - User settings: display name (unique, case-insensitive), Steam ID, language, dark mode.
 - Appearance settings synced across devices: accent color, corner style, wide layout, background and dashboard
   banner from the user's own game art. Only whitelisted presets and Steam app ids are stored, never raw CSS or URLs.
@@ -102,7 +103,6 @@ Runs unit, WebMvc, persistence and integration tests plus the ArchUnit rules. Ne
 
 ## Roadmap
 
-- JWKS endpoint (`/.well-known/jwks.json`), so Core fetches the public key and keys rotate without a redeploy.
 - Short-lived access token plus rotating refresh token in an httpOnly cookie, with CSRF protection. Today the token
   lives 24 h in `localStorage` and cannot be revoked.
 - Account deletion and data export.