feat: apply fix/adapter-health-1-2-9 changes (unified retry, failure_class, RSS enabled)

- adapters/__init__.py: add http_get() unified retry helper + AdapterHTTPError
  + failure_class classification (429/5xx/4xx/error/zero_fetch/ok)
  + last_failure_class on SourceAdapter for pipeline capture
- pipeline.py: ENABLED_SOURCES now includes RSS
  + failure_class rollup in run_log (most severe across all adapters)
  + per-source failure_class in source_stats
- schema.sql: add failure_class column to run_log table

Backport of fix/adapter-health-1-2-9 branch (issues #1, #2, #9).

adapters/__init__.py

@@ -1,11 +1,18 @@
 """Source adapters for AI Research Oracle."""
 
+import urllib.request
+import urllib.error
+import time
 from abc import ABC, abstractmethod
 
 
 class SourceAdapter(ABC):
     """Base class for all ingestion adapters."""
 
+    def __init__(self):
+        # Set by http_get when a request fails (issue #2 classification)
+        self.last_failure_class = None
+
     @abstractmethod
     def name(self) -> str:
         """Source name: 'github', 'arxiv', 'reddit'."""
@@ -15,3 +22,81 @@ class SourceAdapter(ABC):
     def fetch(self, query: str = "", limit: int = 20) -> list[dict]:
         """Return entries matching DB schema fields."""
         pass
+
+
+class AdapterHTTPError(Exception):
+    """Raised by http_get on non-retryable or exhausted HTTP failures.
+
+    failure_class is one of: '429', '5xx', '4xx', 'error'.
+    Consumed by pipeline.py to populate run_log.failure_class (issue #2).
+    """
+
+    def __init__(self, status, failure_class, message=""):
+        super().__init__(message)
+        self.status = status
+        self.failure_class = failure_class
+
+
+def classify_http_status(code):
+    """Map an HTTP status code to a run_log failure_class."""
+    if code == 429:
+        return "429"
+    if 500 <= code < 600:
+        return "5xx"
+    return "4xx"  # 401/403/404 etc = client/config problem, not retried
+
+
+def http_get(url, headers=None, timeout=15, max_retries=2,
+             backoff_base=2, retry_403_ratelimit=False, return_headers=False,
+             owner=None):
+    """GET with a unified retry policy shared by all adapters (issue #1).
+
+    Retries ONLY on 429 and 5xx (transient). 4xx other than 429 are NOT
+    retried (config/client problems). Optional GitHub-style 403 rate-limit
+    handling via retry_403_ratelimit (waits for X-RateLimit-Reset header).
+
+    Returns response body bytes (or (body, headers) tuple if return_headers).
+    Raises AdapterHTTPError on non-retryable or exhausted failures, carrying
+    .failure_class for run_log classification. If owner is provided, sets
+    owner.last_failure_class so the pipeline can record it (issue #2).
+    """
+    def _fail(code, fclass, msg):
+        if owner is not None:
+            owner.last_failure_class = fclass
+        raise AdapterHTTPError(code, fclass, msg)
+
+    req = urllib.request.Request(url, headers=headers or {})
+    last_err = None
+    for attempt in range(max_retries + 1):
+        try:
+            with urllib.request.urlopen(req, timeout=timeout) as resp:
+                body = resp.read()
+                if return_headers:
+                    return body, resp.headers
+                return body
+        except urllib.error.HTTPError as e:
+            last_err = e
+            code = e.code
+            if code == 429 or (code == 403 and retry_403_ratelimit):
+                if attempt < max_retries:
+                    if code == 403:
+                        reset = int(e.headers.get("X-RateLimit-Reset", 0))
+                        wait = (max(reset - int(time.time()), 0) + 1
+                                if reset else backoff_base * (attempt + 1) * 15)
+                        time.sleep(min(wait, 300))
+                    else:
+                        time.sleep(min(backoff_base * (attempt + 1), 30))
+                    continue
+                fclass = "429" if code == 429 else "4xx"
+                _fail(code, fclass, f"HTTP {code} for {url} (exhausted)")
+            if 500 <= code < 600:
+                if attempt < max_retries:
+                    time.sleep(min(backoff_base * (attempt + 1), 30))
+                    continue
+                _fail(code, "5xx", f"HTTP {code} for {url} (exhausted)")
+            # other 4xx (401/403 w/o flag/404) — not retried
+            _fail(code, classify_http_status(code), f"HTTP {code} for {url}")
+        except Exception as e:  # non-HTTP (timeout, DNS, conn reset)
+            _fail(None, "error", f"Request error: {e}")
+    # Defensive: loop should always raise or return
+    _fail(getattr(last_err, "code", None), "error", str(last_err))