feat(store): config table for instance-global app-config overrides

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/store/config_test.go

@@ -0,0 +1,35 @@
+package store
+
+import (
+	"context"
+	"testing"
+)
+
+func TestConfigValues_EmptyThenUpsertOverwrites(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+
+	values, err := db.ConfigValues(ctx)
+	if err != nil {
+		t.Fatalf("ConfigValues (empty): %v", err)
+	}
+	if len(values) != 0 {
+		t.Fatalf("expected no overrides in a fresh DB, got %v", values)
+	}
+
+	if err := db.SetConfigValue(ctx, "session.duration", "168"); err != nil {
+		t.Fatalf("SetConfigValue: %v", err)
+	}
+	// Same key again: upsert must overwrite, not error or duplicate.
+	if err := db.SetConfigValue(ctx, "session.duration", "24"); err != nil {
+		t.Fatalf("SetConfigValue (overwrite): %v", err)
+	}
+
+	values, err = db.ConfigValues(ctx)
+	if err != nil {
+		t.Fatalf("ConfigValues: %v", err)
+	}
+	if len(values) != 1 || values["session.duration"] != "24" {
+		t.Fatalf("expected {session.duration: 24}, got %v", values)
+	}
+}