Paywall detection: swap to Wayback snapshot when article body is gated

Fetch each new article once and measure visible <p> text. If body is below
500 chars (or fetch fails), resolve a Wayback snapshot via the
archive.org/wayback/available API and use that URL for both the og:image
fallback and the posted link. Dedup keys stay derived from the original
URL so paywalled/non-paywalled hits collide as before.

- New: internal/ingestion/article.go (FetchArticleMeta via goquery)
- New: internal/ingestion/wayback.go (ResolveWayback)
- Removed: internal/ingestion/og.go (folded into article.go)
- poller.go: dedup first, then one article fetch, then snapshot fallback

internal/ingestion/wayback.go

@@ -0,0 +1,62 @@
+package ingestion
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/url"
+	"time"
+)
+
+var waybackClient = &http.Client{Timeout: 10 * time.Second}
+
+type waybackResp struct {
+	ArchivedSnapshots struct {
+		Closest struct {
+			Available bool   `json:"available"`
+			URL       string `json:"url"`
+			Status    string `json:"status"`
+		} `json:"closest"`
+	} `json:"archived_snapshots"`
+}
+
+// ResolveWayback asks the Internet Archive's Wayback availability API for the
+// closest snapshot of the given URL. Returns the snapshot URL (https) or ""
+// if no snapshot exists or the request fails.
+func ResolveWayback(articleURL string) string {
+	if articleURL == "" {
+		return ""
+	}
+	api := "https://archive.org/wayback/available?url=" + url.QueryEscape(articleURL)
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	req, err := http.NewRequestWithContext(ctx, "GET", api, nil)
+	if err != nil {
+		return ""
+	}
+	req.Header.Set("User-Agent", userAgent)
+
+	resp, err := waybackClient.Do(req)
+	if err != nil {
+		return ""
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return ""
+	}
+
+	var wr waybackResp
+	if err := json.NewDecoder(resp.Body).Decode(&wr); err != nil {
+		return ""
+	}
+	snap := wr.ArchivedSnapshots.Closest
+	if !snap.Available || snap.Status != "200" || snap.URL == "" {
+		return ""
+	}
+	// Wayback sometimes returns http:// even when https is available.
+	if len(snap.URL) > 7 && snap.URL[:7] == "http://" {
+		return "https://" + snap.URL[7:]
+	}
+	return snap.URL
+}