feat(api): radio handler reads current session vector + threads ContextWeight

Wire contextual scoring end-to-end: add ContextWeight to RecommendationConfig
(struct + Default()), fetch the user's current session vector in handleRadio
via loadCurrentSessionVector (cold-start returns Seed sentinel), and pass it
as the 6th arg to LoadCandidates so ContextualMatchScore flows into Shuffle.

internal/config/config.go

@@ -72,6 +72,7 @@ type RecommendationConfig struct {
 	RecencyWeight       float64 `yaml:"recency_weight"`
 	SkipPenalty         float64 `yaml:"skip_penalty"`
 	JitterMagnitude     float64 `yaml:"jitter_magnitude"`
+	ContextWeight       float64 `yaml:"context_weight"`
 	RecentlyPlayedHours int     `yaml:"recently_played_hours"`
 	RadioSize           int     `yaml:"radio_size"`
 	RadioSizeMax        int     `yaml:"radio_size_max"`
@@ -93,6 +94,7 @@ func Default() Config {
 			RecencyWeight:       1.0,
 			SkipPenalty:         1.0,
 			JitterMagnitude:     0.1,
+			ContextWeight:       2.0,
 			RecentlyPlayedHours: 1,
 			RadioSize:           50,
 			RadioSizeMax:        200,