refactor(shared-go): extract shared auth package from 3 Go services

Create packages/shared-go/authutil/ with two JWT validator implementations:
- JWKSValidator: EdDSA JWKS validation with key caching (extracted from mana-sync)
- RemoteValidator: delegates to mana-core-auth /api/v1/auth/validate (from mana-notify/gateway)

Plus shared types (Claims, User), middleware factories (JWTMiddleware, ServiceKeyMiddleware),
context helpers (GetUser, GetUserID, GetUserRole), and token extraction.

Migrated services:
- mana-sync: internal/auth/jwt.go now wraps authutil.JWKSValidator
- mana-notify: internal/auth/auth.go now wraps authutil.RemoteValidator + ServiceKeyMiddleware
- mana-api-gateway: internal/middleware/jwt.go now wraps authutil.RemoteValidator

All 3 services compile and pass tests. Service-level packages re-export types
for backward compatibility so no consumer code changes are needed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

services/mana-sync/go.mod

@@ -1,15 +1,16 @@
 module github.com/manacore/mana-sync
 
-go 1.23
+go 1.25.0
 
 require (
 	github.com/coder/websocket v1.8.12
-	github.com/golang-jwt/jwt/v5 v5.2.1
 	github.com/jackc/pgx/v5 v5.7.2
+	github.com/manacore/shared-go v0.0.0
 	github.com/rs/cors v1.11.1
 )
 
 require (
+	github.com/golang-jwt/jwt/v5 v5.3.1 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
 	github.com/jackc/puddle/v2 v2.2.2 // indirect
@@ -17,3 +18,5 @@ require (
 	golang.org/x/sync v0.10.0 // indirect
 	golang.org/x/text v0.21.0 // indirect
 )
+
+replace github.com/manacore/shared-go => ../../packages/shared-go