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/github.py

@@ -17,7 +17,7 @@ import urllib.error
 import urllib.parse
 from datetime import datetime, timedelta, timezone
 
-from adapters import SourceAdapter
+from adapters import SourceAdapter, http_get, AdapterHTTPError
 
 
 class GitHubAdapter(SourceAdapter):
@@ -43,40 +43,27 @@ class GitHubAdapter(SourceAdapter):
         return headers
 
     def _request(self, url: str, max_retries: int = 2) -> dict | list | None:
-        """Make a GET request with retry on 403 (rate limit)."""
-        req = urllib.request.Request(url, headers=self._headers())
-
-        for attempt in range(max_retries + 1):
-            try:
-                with urllib.request.urlopen(req, timeout=15) as resp:
-                    data = json.loads(resp.read().decode("utf-8"))
-
-                # Check rate limit headers
-                remaining = int(resp.headers.get("X-RateLimit-Remaining", 0))
-                if remaining <= 5:
-                    print(f"    ⚠ Rate limit low ({remaining} remaining), stopping")
-                    break
-
-                return data
-            except urllib.error.HTTPError as e:
-                if e.code == 403:
-                    # Rate limited — reset time is in headers
-                    reset = int(e.headers.get("X-RateLimit-Reset", 0))
-                    if reset:
-                        wait = max(reset - int(time.time()), 0) + 1
-                        print(f"    ⚠ Rate limited, wait {wait}s")
-                    else:
-                        wait = 30 * (attempt + 1)
-                        print(f"    403 on attempt {attempt + 1}, retry in {wait}s")
-                    time.sleep(min(wait, 300))  # cap at 5 min
-                    continue
-                print(f"    HTTP {e.code} for {url}")
-                return None
-            except Exception as e:
-                print(f"    Request error: {e}")
-                return None
-
-        return None
+        """GET via shared retry helper; 403 rate-limit handled as transient."""
+        try:
+            raw, headers = http_get(
+                url, headers=self._headers(), timeout=15,
+                max_retries=max_retries, retry_403_ratelimit=True,
+                return_headers=True, owner=self)
+        except AdapterHTTPError as e:
+            print(f"    {e.failure_class}: GitHub {url}")
+            return None
+        # Informational: flag if we're close to the unauth rate ceiling
+        try:
+            remaining = int(headers.get("X-RateLimit-Remaining", 0))
+            if remaining <= 5:
+                print(f"    ⚠ Rate limit low ({remaining} remaining)")
+        except Exception:
+            pass
+        try:
+            return json.loads(raw.decode("utf-8"))
+        except Exception as e:
+            print(f"    GitHub decode error: {e}")
+            return None
 
     def _search_repos(self, query: str, sort: str = "stars", order: str = "desc", per_page: int = 30) -> list:
         """Search repositories via GitHub API."""