fix(auth): address PR #758 round-3 final review

Seven findings from the latest review on #758, plus a regression test
catching the substance of the cache-stampede fix:

- verify_id_token: widen id_token annotation to str | None to match
  callers passing nc_token_response.get("id_token")
- extract_user_id_from_token: use JSON-RPC reserved error code -32001
  instead of -1
- _get_cached: per-URL anyio.Lock dict + meta-lock coalesces concurrent
  cache misses into a single IdP fetch (mirrors token_broker.py idiom)
- delete_browser_session: collapse SELECT+DELETE into atomic
  DELETE ... RETURNING user_id (SQLite >= 3.35)
- new test_origin_normalise.py: parametrized port/scheme/host equivalence
  cases for the CSRF Origin guard
- browser_oauth_routes: correct misleading "PR #758 finding 5" cross-
  references (finding 5 was Fernet-key hardening, not CSRF)
- ASProxySession.nonce: make required, drop spurious "legacy session"
  default; reword the in-flight `or None` comment to reflect that
  ASProxySession is purely in-memory
- new test_get_cached_coalesces_concurrent_misses: pins the
  cache-stampede protection — fires 10 concurrent _get_cached calls and
  asserts exactly one HTTP fetch

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/test_oauth_callback_session_cleanup.py

@@ -218,6 +218,7 @@ async def test_as_proxy_rejects_invalid_id_token():
         code_challenge="challenge",
         code_challenge_method="S256",
         requested_scopes="openid",
+        nonce="nonce-rejected",
     )
     _proxy_codes.clear()