Admin: source Articles inspector (verify metrics against real evidence)

New per-row "Articles" button on the Sources table expands a read-only inline
panel of the source's ACTUAL ingested articles — so the automated metrics
(paywall/image/acceptance/duplicate) can be verified against evidence instead of
trusted blind. Distinct from "Check" (which re-samples the LIVE feed for
would-pass quality); this shows what's already in the DB, which is what the table
metrics are computed from.

- Backend: GET /api/admin/sources/{id}/articles?filter=&limit=&offset= (admin,
  read-only). queries.source_articles + source_articles_summary — per article:
  title, url, date, accepted, reason (the "why"), topic/flavor, paywalled
  (domain rule), has_image, duplicate. Summary = counts + source-level paywall
  rule.
- Frontend: expandable panel with a summary header ("27 ingested · 18 accepted
  · … · paywall rule: ON (domain)"), filter chips (All/Accepted/Rejected/No
  image/Duplicates), compact rows with title→link + badges + reason, Load more.

So "100% paywall" or "0% images" becomes clickable evidence: open two articles
to tell a real paywall from a mis-flagged domain, or a true image gap from an
enrichment failure. Test: test_source_articles_inspector. 241 pytest + 11 vitest.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

goodnews/api.py

@@ -1146,6 +1146,24 @@ def create_app() -> FastAPI:
             url = src["feed_url"]
         return _preview_or_502(url)  # safe fetch, no DB connection held
 
+    @app.get("/api/admin/sources/{sid}/articles")
+    def admin_source_articles(sid: int, request: Request, filter: str = "all",
+                              limit: int = 25, offset: int = 0) -> dict:
+        # Read-only inspector: the REAL ingested articles behind a source's metrics,
+        # so paywall/image/acceptance/duplicate signals can be verified against evidence.
+        limit = max(1, min(int(limit), 100))
+        offset = max(0, int(offset))
+        with get_conn() as conn:
+            _require_admin(conn, request)
+            if not conn.execute("SELECT 1 FROM sources WHERE id = ?", (sid,)).fetchone():
+                raise HTTPException(status_code=404, detail="source not found")
+            arts = queries.source_articles(conn, sid, filter, limit, offset)
+            return {
+                "articles": arts,
+                "summary": queries.source_articles_summary(conn, sid) if offset == 0 else None,
+                "has_more": len(arts) == limit,
+            }
+
     # --- Source candidates (supervised add-a-source pipeline) ----------------
 
     def _candidate_dict(row) -> dict: