Add per-source language filter

When a source sets language = "en", drop items whose per-item
language tag is present and doesn't prefix-match. Items without
a language tag pass through unchanged. Politico Europe is the
motivating case — same headlines appear in en, fr, and de.

internal/ingestion/poller.go

@@ -3,6 +3,7 @@ package ingestion
 import (
 	"context"
 	"log/slog"
+	"strings"
 	"sync"
 	"time"
 
@@ -109,6 +110,17 @@ func (p *Poller) pollOnceWithErr(ctx context.Context, src config.SourceConfig) e
 			continue
 		}
 
+		// Drop items in languages this source isn't configured to surface.
+		// Items without a language tag pass through — only filter when the
+		// feed explicitly marks an item with a non-matching language.
+		if src.Language != "" && items[i].Language != "" &&
+			!strings.HasPrefix(items[i].Language, strings.ToLower(src.Language)) {
+			slog.Info("dropping story (language filter)",
+				"guid", items[i].GUID, "source", src.Name,
+				"item_lang", items[i].Language, "want", src.Language)
+			continue
+		}
+
 		// Dedup checks first — canonical and headline use the ORIGINAL URL so
 		// they stay stable whether we end up swapping to a Wayback snapshot.
 		originalURL := items[i].ArticleURL