refactor(signal_v2): narrow stock_client exception catch + remove dead code

Code quality review fixes:
- _cached_request: catch httpx.HTTPError instead of bare Exception
  (avoid swallowing KeyboardInterrupt / asyncio.CancelledError)
- _request_with_retry: remove unused last_exc variable + dead post-loop
  raise paths. Final sentinel raise preserved for mypy.

6 tests still pass. Deferred to Phase 7 backlog: cache concurrency
coalescing + __aenter__/__aexit__ context manager support.

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

signal_v2/stock_client.py

@@ -84,7 +84,7 @@ class StockClient:
             data = await self._request_with_retry(method, path, **kwargs)
             self._cache[cache_key] = (data, time.monotonic())
             return data
-        except Exception:
+        except httpx.HTTPError:
             # Stale fallback: serve old cached value if exists
             if cache_key in self._cache:
                 stale_data, stale_ts = self._cache[cache_key]
@@ -98,7 +98,6 @@ class StockClient:
     async def _request_with_retry(self, method: str, path: str, **kwargs) -> dict:
         url = f"{self._base_url}{path}"
         headers = self._auth_headers()
-        last_exc: Exception | None = None
         for attempt in range(_MAX_ATTEMPTS):
             try:
                 response = await self._client.request(
@@ -111,18 +110,15 @@ class StockClient:
                     response.raise_for_status()
                 response.raise_for_status()
                 return response.json()
-            except httpx.TimeoutException as e:
-                last_exc = e
+            except httpx.TimeoutException:
                 if attempt < _MAX_ATTEMPTS - 1:
                     await asyncio.sleep(2**attempt)
                     continue
                 raise
-            except httpx.HTTPStatusError as e:
-                last_exc = e
+            except httpx.HTTPStatusError:
                 raise
-        if last_exc is not None:
-            raise last_exc
-        raise RuntimeError("retry exhausted")
+        # Unreachable: every iteration either returns or raises
+        raise RuntimeError("_request_with_retry exhausted loop without raising")
 
     def _auth_headers(self) -> dict[str, str]:
         return {"X-WebAI-Key": self._api_key}