fix(adapters): shared retry helper + run_log failure_class + enable RSS (issues #1 #2 #9)

- adapters/__init__.py: add http_get() unified retry (429/5xx only, max 2
  attempts, capped exp backoff) + AdapterHTTPError carrying failure_class;
  SourceAdapter.last_failure_class set on failure for pipeline capture.
- arxiv/github/huggingface/hackernews/reddit: route HTTP through http_get.
  Preserves GitHub 403 rate-limit retry and Reddit 403/429 fast-bail.
- schema.sql + pipeline.py: add run_log.failure_class column; rollup most-
  severe class across sources (5xx>4xx>429>error>zero_fetch>ok).
- pipeline.py: ENABLE RSS in ENABLED_SOURCES (was registered, disabled).
- RSS smoke test surfaced 3 broken feeds (anthropic 404, googleai 404,
  metaai 301) — left as-is, captured in feed_failures; URL fix is separate
  discovery task, not guessed.

Verified: full dry-run fetches all 6 sources; github live fetch OK;
Reddit 429 fast-bail preserved; no import/syntax errors.

adapters/huggingface.py

@@ -34,7 +34,7 @@ import urllib.request
 import urllib.error
 from datetime import datetime, timedelta, timezone
 
-from adapters import SourceAdapter
+from adapters import SourceAdapter, http_get, AdapterHTTPError
 
 
 class HuggingFaceAdapter(SourceAdapter):
@@ -84,24 +84,19 @@ class HuggingFaceAdapter(SourceAdapter):
         return headers
 
     def _request(self, path: str, max_retries: int = 2) -> list | dict | None:
-        """Make a GET request to the HF API."""
+        """GET via shared retry helper (retries 429/5xx)."""
         url = f"{self.BASE}{path}"
-        req = urllib.request.Request(url, headers=self._headers())
-
-        for attempt in range(max_retries + 1):
-            try:
-                with urllib.request.urlopen(req, timeout=20) as resp:
-                    return json.loads(resp.read().decode("utf-8"))
-            except (urllib.error.HTTPError, urllib.error.URLError) as e:
-                if attempt < max_retries:
-                    time.sleep(3 * (attempt + 1))
-                    continue
-                print(f"    HF API error: {e}")
-                return None
-            except Exception as e:
-                print(f"    Request error: {e}")
-                return None
-        return None
+        try:
+            raw = http_get(url, headers=self._headers(), timeout=20,
+                           max_retries=max_retries, owner=self)
+        except AdapterHTTPError as e:
+            print(f"    {e.failure_class}: HF {path}")
+            return None
+        try:
+            return json.loads(raw.decode("utf-8"))
+        except Exception as e:
+            print(f"    HF decode error: {e}")
+            return None
 
     def _is_ai_relevant(self, model: dict) -> bool:
         """Check if a model/dataset is AI/ML relevant.