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

go.mod

@@ -3,6 +3,7 @@ module pete
 go 1.25.0
 
 require (
+	github.com/PuerkitoBio/goquery v1.12.0
 	github.com/mmcdole/gofeed v1.3.0
 	gopkg.in/yaml.v3 v3.0.1
 	maunium.net/go/mautrix v0.28.0
@@ -11,7 +12,6 @@ require (
 
 require (
 	filippo.io/edwards25519 v1.2.0 // indirect
-	github.com/PuerkitoBio/goquery v1.12.0 // indirect
 	github.com/andybalholm/cascadia v1.3.3 // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/google/uuid v1.6.0 // indirect