refactor(nws): remove internal AsyncLimiter rate limiting

The NWSAdapter had an internal AsyncLimiter that duplicated the
supervisor's rate-limit guarantee. When cadence changed, only
state.adapter.cadence_s was updated, not the internal limiter,
causing the cadence-decrease bug.

Since the supervisor already enforces rate limiting via
last_completed_poll + cadence_s scheduling, the adapter-level
limiter was redundant and caused the 30-second blocking observed
in diagnostic logs.

Removes:
- aiolimiter import
- self.cadence_s attribute (unused elsewhere)
- self._limiter creation
- async with self._limiter context in _fetch_alerts

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/central/adapters/nws.py

@@ -10,7 +10,6 @@ from pathlib import Path
 from typing import Any
 
 import aiohttp
-from aiolimiter import AsyncLimiter
 from tenacity import (
     retry,
     stop_after_attempt,
@@ -199,11 +198,9 @@ class NWSAdapter(SourceAdapter):
         cursor_db_path: Path,
     ) -> None:
         self.config = config
-        self.cadence_s = config.cadence_s
         self.states = set(s.upper() for s in config.states)
         self.cursor_db_path = cursor_db_path
         self._session: aiohttp.ClientSession | None = None
-        self._limiter = AsyncLimiter(1, config.cadence_s)
         self._db: sqlite3.Connection | None = None
 
     async def startup(self) -> None:
@@ -329,7 +326,6 @@ class NWSAdapter(SourceAdapter):
     )
     async def _fetch_alerts(self) -> tuple[int, dict[str, Any] | None, str | None]:
         """Fetch alerts from NWS API with conditional request."""
-        async with self._limiter:
         if not self._session:
             raise RuntimeError("Session not initialized")