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.

schema.sql

@@ -22,6 +22,8 @@ CREATE INDEX IF NOT EXISTS idx_entries_category ON entries(category_tags);
 -- Partial failures (e.g. Reddit rate-limited) are detectable here, not hidden
 -- as a "complete" run. Also enables future pruning decisions (entries older
 -- than N days with no re-fetch can be archived).
+-- failure_class (issue #2): one of 4xx / 5xx / 429 / zero_fetch / ok, derived
+-- from the real HTTP response via adapters.http_get, not guessed after the fact.
 CREATE TABLE IF NOT EXISTS run_log (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     run_time TEXT DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now')),
@@ -29,6 +31,7 @@ CREATE TABLE IF NOT EXISTS run_log (
     total_stored INTEGER DEFAULT 0,
     sources_ok TEXT,       -- JSON list of sources that succeeded
     sources_failed TEXT,   -- JSON list of sources that errored/skipped
+    failure_class TEXT,    -- 4xx / 5xx / 429 / zero_fetch / ok
     notes TEXT
 );