Rework history: opened + replaced only, with per-item removal

History was logging every article merely displayed, which made it noise. Split
the two concepts cleanly:
- "displayed" (seenIds) still tracks everything shown, but only to stop Replace
  recycling stories — it no longer feeds history.
- "history" now records only deliberate events: articles the user OPENED (card
  click) or ones they REPLACED away (recoverable accidental swaps).

Also: per-item removal (× in the History panel; DELETE /api/history/{id}), and
when signed in the panel shows the account (cross-device) history. First-sign-in
import now folds the meaningful history (not everything shown). Copy updated.
115 tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

goodnews/api.py

@@ -497,6 +497,17 @@ def create_app() -> FastAPI:
             conn.commit()
         return {"ok": True}
 
+    @app.delete("/api/history/{article_id}")
+    def remove_history(article_id: int, request: Request) -> dict:
+        with get_conn() as conn:
+            user = _require_user(conn, request)
+            conn.execute(
+                "DELETE FROM user_history WHERE user_id = ? AND article_id = ?",
+                (user["id"], article_id),
+            )
+            conn.commit()
+        return {"ok": True}
+
     @app.post("/api/import")
     def import_local(body: ImportBody, request: Request) -> dict:
         """Fold this device's anonymous history/saved into the account (one-time)."""