Simplify ts-core auth.ts; fix bun-types resolution in monorepo workspace

auth.ts: dropped the legacy HS256-shared-secret verification path, keeping
only the JWKS path Supabase recommends now — one code path instead of two.

Also fixes a real bug: @types/bun's ambient types didn't resolve inside a
bun workspace (its internal 'bun-types' reference can't hoist into a nested
package's node_modules the same way it does in a flat install). Depending on
bun-types directly instead of @types/bun fixes it — verified with a clean
'bun run typecheck'.

packages/ts-core/package.json

@@ -24,7 +24,7 @@
     }
   },
   "devDependencies": {
-    "@types/bun": "^1.1.14",
+    "bun-types": "^1.1.14",
     "typescript": "^5.6.3",
     "elysia": "^1.1.26"
   }