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/config/config.go

@@ -70,6 +70,10 @@ type SourceConfig struct {
 	PollIntervalMinutes int    `toml:"poll_interval_minutes"`
 	DirectRoute         string `toml:"direct_route"`
 	Enabled             bool   `toml:"enabled"`
+	// Language, when set, drops feed items whose per-item language tag is
+	// present and does not match (prefix). Useful for multilingual feeds
+	// like Politico Europe that publish English + French side-by-side.
+	Language            string `toml:"language"`
 }
 
 func Load(path string) (*Config, error) {