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/reddit.py

@@ -24,7 +24,7 @@ import xml.etree.ElementTree as ET
 from datetime import datetime, timezone
 from html import unescape
 
-from adapters import SourceAdapter
+from adapters import SourceAdapter, http_get, AdapterHTTPError
 
 
 class RedditAdapter(SourceAdapter):
@@ -101,33 +101,27 @@ class RedditAdapter(SourceAdapter):
         return False
 
     def _fetch_rss(self, subreddit: str) -> list[dict]:
-        """Fetch RSS feed for a subreddit."""
-        url = f"https://www.reddit.com/r/{subreddit}/hot/.rss?limit=50"
-        req = urllib.request.Request(url, headers={"User-Agent": self.user_agent})
+        """Fetch RSS feed for a subreddit (shared retry helper).
 
-        for attempt in range(2):  # max 2 attempts, fail fast
-            try:
-                with urllib.request.urlopen(req, timeout=10) as resp:
-                    xml_data = resp.read().decode("utf-8")
-                break
-            except urllib.error.HTTPError as e:
-                if e.code in (403,):
-                    print(f"    RSS blocked (HTTP {e.code}) for r/{subreddit}")
-                    return []
-                if e.code == 429:
-                    if attempt == 0:
-                        time.sleep(2)  # single retry with short backoff
-                        continue
-                    print(f"    RSS rate-limited for r/{subreddit}, skip")
-                    return []
-                print(f"    RSS HTTP {e.code} for r/{subreddit}")
-                return []
-            except Exception as e:
-                print(f"    RSS error r/{subreddit}: {e}")
-                return []
-        else:
-            print(f"    r/{subreddit}: still rate limited, skip")
+        Preserves prior fast-bail: 403 -> immediate []; 429 -> single 2s
+        retry then []; 5xx -> helper retry then []. Not slower than before.
+        """
+        url = f"https://www.reddit.com/r/{subreddit}/hot/.rss?limit=50"
+        try:
+            raw = http_get(url, headers={"User-Agent": self.user_agent},
+                           timeout=10, max_retries=1, backoff_base=2, owner=self)
+        except AdapterHTTPError as e:
+            if e.status == 403:
+                print(f"    RSS blocked (HTTP 403) for r/{subreddit}")
+            elif e.status == 429:
+                print(f"    RSS rate-limited for r/{subreddit}, skip")
+            else:
+                print(f"    RSS {e.failure_class} for r/{subreddit}")
             return []
+        except Exception as e:
+            print(f"    RSS error r/{subreddit}: {e}")
+            return []
+        xml_data = raw.decode("utf-8")
 
         # Parse Atom XML
         entries = []
@@ -303,33 +297,27 @@ class RedditAdapter(SourceAdapter):
         return tags
 
     def _fetch_rss(self, subreddit: str) -> list[dict]:
-        """Fetch RSS feed for a subreddit."""
-        url = f"https://www.reddit.com/r/{subreddit}/hot/.rss?limit=50"
-        req = urllib.request.Request(url, headers={"User-Agent": self.user_agent})
+        """Fetch RSS feed for a subreddit (shared retry helper).
 
-        for attempt in range(2):  # max 2 attempts, fail fast
-            try:
-                with urllib.request.urlopen(req, timeout=10) as resp:
-                    xml_data = resp.read().decode("utf-8")
-                break
-            except urllib.error.HTTPError as e:
-                if e.code in (403,):
-                    print(f"    RSS blocked (HTTP {e.code}) for r/{subreddit}")
-                    return []
-                if e.code == 429:
-                    if attempt == 0:
-                        time.sleep(2)  # single retry with short backoff
-                        continue
-                    print(f"    RSS rate-limited for r/{subreddit}, skip")
-                    return []
-                print(f"    RSS HTTP {e.code} for r/{subreddit}")
-                return []
-            except Exception as e:
-                print(f"    RSS error r/{subreddit}: {e}")
-                return []
-        else:
-            print(f"    r/{subreddit}: still rate limited, skip")
+        Preserves prior fast-bail: 403 -> immediate []; 429 -> single 2s
+        retry then []; 5xx -> helper retry then []. Not slower than before.
+        """
+        url = f"https://www.reddit.com/r/{subreddit}/hot/.rss?limit=50"
+        try:
+            raw = http_get(url, headers={"User-Agent": self.user_agent},
+                           timeout=10, max_retries=1, backoff_base=2, owner=self)
+        except AdapterHTTPError as e:
+            if e.status == 403:
+                print(f"    RSS blocked (HTTP 403) for r/{subreddit}")
+            elif e.status == 429:
+                print(f"    RSS rate-limited for r/{subreddit}, skip")
+            else:
+                print(f"    RSS {e.failure_class} for r/{subreddit}")
             return []
+        except Exception as e:
+            print(f"    RSS error r/{subreddit}: {e}")
+            return []
+        xml_data = raw.decode("utf-8")
 
         # Parse Atom XML
         entries = []