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).

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
 );